My paddling partner has the 76x without buying the marine chart card and it works fine. I like the compact size of the etrex20, and it has some marine map info and coastal tides installed like the 76x.
I would keep either in a clear dry bag and not fully trust the 7 waterproof rating on saltwater. As for floating I just add a foam float to the lanyard to float my camera and one to float my GPS.
Also there are ways to make custom maps from marine charts and load them to a GPS.
Reflective Hull Decals
PFD's (Life Jackets)
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