If your going to try and kill it with water every time you go out and have water up to your seat belt, I wouldnt worry about it, sounds like a stupid Idea to do that in my book but, do what you want, you could put a sprint car seat in there like a butler built or somthing, then you could put a 5 point restraint system in it. You'll some day learn that muddy water is pretty fun to a certian point, but your grow up one day and learn that its not as cool as you think, anyone can drive into a deep rump roast mud hole at 45 MPH and put the fire out, but wheeling on a real rail is a lot more fun than being swamped in your car, then going swiming to hook it up and get it out.
bottom Line in Plastic Seats are a huge FAIL. Leave what you got in it and deal with it. I would fix the trans and front axle before I went wheeling again though.........
Jeff