I haven't used a hand held GPS from a boat yet, but I kayak the 10,000 islands ALOT (my favorite place for overnighters). I can tell you that my previous GPS, the Garmin 76CSX, worked above and beyond my standards. It was easy to use, had depth marking, I could change in between Blue Charts and Topos for use on land, and it was DURABLE! I can say that I abused the thing. I dropped it, neglected to properly clean it, and banged it up in general. The thing would not give up.
When they say water proof, they mean it. It wasn't one of these "splash proof' GPSs where they CAN get wet, it was water proof. There were many times when it would fall in the water and I wouldn't notice it and it would be draged for 5 minutes in water. I would pluck it out, wipe the sceen clear, and countine on using it. My vote no doubt goes for the 76CSX.
I now have a 78sc and that has been working well for the last two paddles. Hope it workes as well as the 76CSX. ;D