I carry my same "more $$ =/= better" methodology into airplanes too... I'm an EdgeTX/Radiomaster guy.
You don't necessarily need a waterproof RX, but you should have the RX in a waterproof radio box, or you can do like some people, and wrap it in something waterproof. Balloons are popular, as are non-lubricated condoms! All of my boats have the RX's in a waterproof radio box, but I keep a few condoms in my field box just in case.
And really, wheel/surface type radios are obviously more popular with boat guys (myself included), but you could also use your plane TX if you wanted to. I've driven boats with my stick type TX, and don't really care for it as I find wheel radios more intuitive, but if you've already got it, there's no reason you can't use it. Just pick up a cheap 2/3/4 channel Spektrum RX and you'd be good to go.