1 Start the level regularly inside the tank (do not leave it for now)

2 Open the console then type nvngod (now you're actually on god-mode)

3 Keep playing on god-mode inside the same tank until you reach a big wall that you can hit on the left or on the right with a single cannon shot; AIM ON THE RIGHT ONLY and then keep moving until you reach the dam. Now you can also leave the tank

4 After opening the passage, go for it (no matter with or without the tank) but do not enter, turn off god-mode instead (open console and then type nvngod again), clear the area if you need it, then procede. Profit!

In few words, the problem will occur if you choose to pass through the left side of the wall (read point 3) or by taking damage anytime before and soon after you've reached the dam.