I thought a BT module or WIFI board would work fine sealed in a submarine.
No they would not.
Radio communication reflects off water?
No the water changes the wavelength of EM waves. It makes them longer, that means the antenna are no longer tuned up correctly. Plus the fact that water absorbs the radio waves. Submarines have to trail very lone wires, in the order of a few miles to compensate for the change when receiving signals with only a Km or so wavelength. With UHF you have no chance.
Try it and see.