If you don't need chartplotting, any waterproof gps will do. You just need to pre enter your waypoints before. This is what I tend to do but I've learnt to sail in the late 80's (pre-GPS) so I am probably old school.
I think that my Dakota 20 would read a marine map (to be bought and quite pricey), it works well as an audax GPS so you may look at this. Nevertheless to have a true chartplotter you need a quite big screen which would make it useless for cycling.
In short most gps people use for cycling would be useful on a boat but the contrary wouldn't be true.
Next time we meet, you can play with my Dakota.