To answer your question that rx is fine for electric boats, the dual antennae helps to eliminated signal loss in a marine environment that can happen with all 2.4 systems, never experienced it but the signal can be reflected off the water, also the signal doesn't penetrate water so make sure you antennae are above the water line.
Remember also that not all Spektrum receivers will not work with all Spektrum systems, I always use the marine rx so off the top of my head I can't remember what works with what, Look for the surface and marine logos on the rx and you are ok.
I'm sure you already know those rx are water resistant.