Uhh i have been into Rc for quite some time. From what i know i don't think there are any cars that switch from 2wd to 4wd. Maybe the traxxas summit but i forgot. Waterproof and nitro don't really mix as well becuase I think water can get in the engine through the intake. You could try electric if you would like. Those can be like almost submerged under water.
anything is possible but it would be cheaper to buy a brand new nitro if you were planning on converting it. It's difficult to do and probalby very time consuming. And many electric cars have plastic frames whereas nitro cars generally have aluminum or metal frames. And nitro cars are a little beefier too.