First do walkthrough camera mode. Go downstairs and walk to the stern. Go to the starboard side and you will notice a ladder there. Stand in front of it, look up, and hold Up. You will start to climb the ladder. Then after a few seconds the camera will start to move faster, so you think "Hey, I am up!". But you are not. You fall down, into the water, and can just move there like in walkthrough mode. I warn you, be careful with the right mouse button!
Oh yeah, you see, my Red Jet 4 has holes, just on the waterline. Strangely I did not sink.