Documentation
 

I would love to start filming beyond the sea/ under water, because I am very interested into the water world and making some kind of documentary work.
So I wanted to ask which kind of cameras and other stuff I do need, to make sure that everything works out fine and what cameras are not being destroyed by being contacted with water?

Proper enclusures, really. You can find one for almost every camera. Surf BandHphoto. I've literally shot with an FX1 ina big ziplock freezer bag before.
I don't recomend that.
lol

First, you need to be a good diver with great buoyancy control and situational awareness. When you shoot underwater you have two jobs; 1. stay alive, 2. shoot good video. Both require your utmost attention.

Second do a google search for underwater housing manufacturers. Pick the housing first, then the camera as u/w housings are made for specific cameras and not universal. It also wouldn't be a bad idea to rent or buy a used set up first before you plunge headlong into an expensive endeavor.

Finally, watch TV and make a list of all the u/w documentaries you come across. There won't be many. It's a very small field.