Originally Posted by mfr02

It needs to be one that will work with your receiver.
In the days of AM and FM sets, this was fairly simple, but modern 2G4 sets tend to run on software protocols that are unique to their own company at the low cost end. At the pricier end, there are multi protocol sets. Swapping a transmitter might well involve a receiver swap as well.
I don't think anybody makes a waterproof transmitter. They tend to need too many mechanical devices poking through the case, (sticks, switches) and things like access hatches for the batteries. Prevention is the answer - a cover, either a shop-bought padded glove or the emergency plastic bag or a jacket with enough room for both you and the transmitter, and a neck strap.
Ron's experience with the immersed transmitter was probably in fresh water, it would probably not have survived even a brief dunk in the salt water lake that I sail without very fast recovery and attention.