I might be able to help, I just bought a Garmin GPSMAP 76, it's the older monochrome screen version that they still sell. it is waterproof and also floats.
I wish I could have played more with the newer version of this same GPS, the newer color models have more memory built in and you can expand the memory.
I started with the built in base map, but it sucked bad for the lakes around my house, so I bought the 100$ US lakes upgrade, and it was worth the upgrade, but you might as well add that to the cost of the GPS, it does have most the lakes in the U.S. but my memory is only 8megs (haha) built in, so I can only load about 2 regions at a time, enough to cover 3 lakes around here, that's not too bad really, and you can always change it up if going somewhere new.
The lack of the color screen does not bother me so much, but I do wish the zoom in and out was smoother faster, you have to wait for the screen to update every time you zoom in or out, maybe the newer units are faster and better at this? now adays with interfaces on cell phones etc, being so smooth and easy to use, all these GPS units seem slow and ugly to me.
And now that I think about it I am not too sure how big a deal it is that it floats, I might have been able to get a cheaper Etrex model instead of this model, because I figured the floating part was worth the extra money, but ever since I mounted it on my ski, the mount itself is heavy enough it would sink anyway if it fell in the water with the mount attached.
I mounted mine 2 different ways, I have a 2009 FX HO, and there is a cup holder, so I used and RAM Mount cup holder mount that really did work fine, it holds it in there pretty good and the goose neck on it is pretty hard to move around, I was impressed with it really. I then tried another way, I have a water proof compartment right above the left side of the handle, that has a large plastic screw cap on it, I bought a couple extra caps for cheap, then mounted a adjustable RAM mount on to that cap, It works really well, but I am afraid when I can taking the cap off sometime I will drop it in the water, so on both mounts I feel like I still need a leash hooked to the ski encase I drop it.
One thing I can say is that so far I have really got a better overall shape of the lake when riding in it, and also I marked all the restaurants and friends houses on the lake so I can check and see what I am close to, it's pretty nice really, but I can see where after awhile I the GPS is going to teach me the lake good enough I might not need it as much after a bit, but still good when I go to new locations.