Secret #1
After the boat sinks, swim back near the beggining of the level:

Dive down and you'll see a huge hole in the ground:

Swim all the way to the bottom and you'll see an opening in one of the walls (it's a long way down, be mindful of your air gauge):

Inside, you'll get the secret. Note that there's a Missile Launcher in the back, along with a teleporter. Take it to get to another part of this secret, with some extra goodies. Blow up the crack in the wall to get out, but do it quick! The room will collapse soon after you blow your way.


Secret #2
Back to the main track, you'll go through some kind of house, with a table, painting, and a training dummy. If you step on the table, it'll lower. Once it does, the painting nearby slides to reveal the secret.


Secret #3
On the corridor leading to the Gold key door, there are 2 flame statues. Behind one of them is a wooden panel. Break the statue and open the panel for the secret.


Extra
Nearby Secret #3, there's a crack on the wall, hiding a Fortune Cookie!


Secret #4
In the waterfall area, climb to its top, crouch and move to the back wall to get the secret (a Rail Gun).


Secret #5
On your way to the Gold Key, look up and you should notice a gong. Ring it to reveal a hidden compartment nearby.


Secret #6
Inside the room locked by the Gold key, there is a big painting in the wall. Jump inside it to get the secret.


Secret #7
In order to get the secret, you'll need to jump on two statues. The first one is located near the waterfall area, before the corridors that lead to the Gold and the Silver key doors:

Later, when traversing the corridor with a shooting statue, there'll be a small pond with the second statue. Lower the statue in this area and the nearby door should open.

Secret #1
Follow the river in front of you when the level begins. As soon as you fall down, turn around and there should be a small hole partially hiddent by vines with an armor inside. That's the first secret.


Secret #2
Continue following the river until you spot a Buddha statue and a gong. Behind the statue is a door. Shoot the gong and the door will open.


Secret #3
Continue the path past the area with the Gold key door and in the next area, right in front of one of the Bronze key doors will be a big crack in the wall. Explode it to reveal the secret.


Secret #4
This secret is a bit tricky to get. It's located in the room with the Gold key. Cross the traps and when you're in the same platform where the Gold key is, look to the banners of the corridor you just passed. From the start, one will have a Heat Seeker Card. The second, somewhat hidden behind the debris, will have a Nuclear Warhead. In order to reach it, jump on top of the banner across it, and run-jump to the other banner (a strafe jump is recommended). Once you make it, collect the warhead for the secret.


Secret #5
When going to the Silver key door, look to the right of the waterfall and you should notice a hole partially hidden by vines. Jump into the hole to get the secret.

Secret #1
Nearby the Red key door is a pond with a Guardian Head and some dead trees. Near the entrance to this pond is a crack in the wall. Explode it to reveal the secret.


Secret #2
On the boat with the Red key, there's a waterfall nearby. Jump from the tip of the boat to the area on top of the waterfall for the secret.


Secret #3
Before fixing the boat, go to the back area with the Gold key door. Across from it is a stack of boxes shadowing a crack in the wall. Blow it up to reveal the secret.


Secret #4
After fixing the boat, keep your eyes on the left. Once you spot a small waterfall, stop, leave the boat and jump into it to get the secret.


Secret #5
From the waterfall with the Bronze key, there's a hole on a far wall, more or less in the direction of the Red key boat. Navigate and enter this hole for the secret.

If you're having trouble finding it, another good place to spot it is near the Gold key door. It's almost a straight line from the nearby stack of the crates to the hole:

Secret #1
Near the Silver key door is a garden with a big crack in a wall. Blow it up to reveal the secret.


Extra
Near the entrance to the corridor that leads to the Gold key area, there's a gong. Shooting it will lower a nearby pillar with a Ripper Heart inside!


Secret #2
In the Gold key area, there's a Buddha statue in the middle of a small pond. Step on its head to lower it. This will lower a nearby wall, revealing the secret.


Secret #3
Right after opening the Silver key door, there will be a panel with some candles in front of you. Jump walk into the panel for the secret.


Secret #4
From the last secret, walk down the stairs, and then climb the steps to the left. Once at the top, to the left should be a gong. Shoot it and it'll reveal a compartment just beside the wall behind you, near the fireplace.


Secret #5
Near the last secret, to the left of the fireplace, is another small panel with candles around it. Use the panel to make it slide and reveal the secret.


Secret #6
Still in the same room as the last 2 secrets, across from the Bronze key door is a gray wall decoration. Use it and a compartment will open to your left, which is the secret.

Secret #1
Right at the beggining of the level, to your left, is a big panel. Just jump into it to get the secret.


Extra 1
I consider the way forward in this level very cryptic, so I'll add some help here. Near the Secret #1 is an open space where a statue could be. Opening the wall in the back reveals a sewer-like area. Once inside, enter the nearby waterfall to get the Red key which will allow you to proceed forward!


Extra 2
In the room opened by the Red key is a gong. Shooting it opens all 4 panels in the corners of the room with goodies inside of each!


Extra 3
As you make your way through the level, you'll notice a Full Kevlar Armor hanging atop a wooden structure and several shurikens already pinned to the wall. Actually, you can climb them to reach the armor!


Secret #2
In the room opened by the Silver key, find the Gold key door. Right beside it is a big crack. Blow it to reveal the secret.


Secret #3
Inside the room with the Gold key (partially hidden in a small garden), and nearby it, is another crack in the wall. Once again, blow it up to reveal the secret.


Secret #4
In the room opened by the Gold key (where you'll collect the Bronze key), is a large panel to the right. Open it to reveal the secret.


Secret #5
Right beside the end-level symbol is yet another crack in the wall. Explode it to reveal the last secret.

Secret #1
From the beggining of the level, cross the bridge and enter the room to your right (it has a large pillar in the middle). To the left is a statue and beside it a crevice. This one in particular has it back wall covered in vines (whereas the back wall of the crevice opposite to this one isn't). Open it to reveal the secret.


Secret #2
In the room with the Gold key door, look in the small space between the stairs and the wall with the painting to spot a switch. Flick it to make the painting slide and reveal the secret. Also, note that the secret level entrance is behind bars: we'll be removing those later on.


Secret #3
Use the Gold key and enter its room. Inside it, there is a big staircase. Descend it and keep going until you hit a wall. Look to the left and you should see a small hole. Jump into it to get the secret.


Secret #4
From the last secret, jump out and go towards the small waterfall. From there, look up and to the right, towards the way you're supposed to go to spot a large crack in the wall. Blow it and a big concrete piece falls down. If you go to it, you'll notice that it's a part of a stair, which you can jump into. Do so to get the secret.


Secret #5
After flooding the area with the Silver key, if you go up where the Silver key was, you'll find some goodies. Submerge again and continue forward. When possible emerge once again. This small room is the secret.


Secret #6
From the last secret, continue your way down the water and emerge at the Red key door room. Look beside the Bronze key door to spot a pillar with a dragon statue on top. If you stand on top of it, the pillar will lower. This will reveal a secret, in a wall to your left and up. Jump to it to get the secret (which will teleport you on a top portion of the room with the Serpent sub-bosses).


Secret Level
Once you acquire the Red key and open its corresponding door, instead of going to the end level switch, look near the shadow of the nearby cherry tree to spot some vines. These are actually climbable and, once you do, you'll see a switch nearby. Activate it to lower the barriers in Secret #2 granting you access to the Secret Level.

Secret #1
As soon as the level start, you're on the tip of a boat. Turn around, facing the big waterfall and jump into it to get the first secret.


Secret #2
Right after using the Red key and enter the room, look to your left and up: above the small fire is a switch. Activating it will make a nearby snake face wall slide revealing the secret (be careful of the ninja guarding it).


Secret #3
After you collect the Silver key, look to your right and you should spot a small ledge and a hole in the wall. Jump to the ledge and then to the hole to get the secret.


Secret #4
From the last secret, drop down to the water and proceed thorough the level. When you face the lava area, look around for a ladder:

Climb it (there's a Fortune Cookie inside), and notice that a part of the wall is darker. Open it to reveal the secret.


Secret #5
Continue through the level and right after climbing some stairs, there'll be a round door nearby. All the way across from it is a gong. Shoot it to open the door and reveal the secret.

Secret #1
At the beggining of the level, near the Gold key door, notice that there's a small crevice to the right of the door, whereas the left spot is closed. Open this spot to reveal the secret.


Secret #2
After opening the Red key door, look to the right of the spinning trap: the wall can be opened to get into a secret area.


Secret #3
From the last secret, instead of going to the door to exit the room, go to the room in-between the two spinning traps. There should be some kind of furniture (a sofa perhaps?), which can be opened to reveal the secret (beware of the ninja inside).


Extra
In the room after the spinning traps, notice the bookcases: the middle one contains no books. If you press it, it will slide revealing a Rail Gun!


Secret #4
After climbing the wooden ladder (which ends right in the face of a fireball trap), look behind you to spot a crack in the wall in one of the corners. Blow it up to reveal the secret (which contains another crack that takes you back near the level start).


Secret #5
After opening the Silver key door and evading the moving spike wall trap, before jumping to the moving platforms, look to the edge of the level. It's possible to walk along the border, around to the back of this area. There sits an island full of goodies, which is also the secret area.

Secret #1
BE VERY CAREFUL ABOUT THIS SECRET!!!!!!!! It's the only one in SW that can be rendered unreachable if done wrong!
As you begin the level, go to the left and, afteryou make a right turn, you should spot a gas truck, some gas canisters beside it, and some female ninjas on top:

DO NOT BLOW UP THE TRUCK!!!. Instead, kill the ninjas using non-explosive weapons. After that, go to the back of the truck and press against the wall with the "Janitor" sign on top to open the secret.


Secret #2
After collecting the last secret, finally you can blow up the truck to proceed in the level. Go through the water and, after emeging back on land, notice the two bathroom stalls near you. Open the right one and press the flush button. You should hear a door slide. It's located to the right of the stall, and inside is the secret.



Secret #3
After going up the stairs that lead to the Blue key door, notice the Buddha statue in an altar. Use an explosive weapon to blow the altar up (it's not just the statue, it's the whole altar), to create a big hole in the wall, revealing the secret.


Secret #4
From the last secret, open the Blue key door, and then go straight, circling the pillar right in front of you. In the back wall is a crack (which can be though to spot, due to the low lights in the room). Blow it up to reveal the secret.


Secret #5
After using the Yellow key and entering the room, look to your right to see a towel hanging on the wall. Press it to make a nearby portion of the wall slide revealing the last secret.