I've had a Garmin GPSmap 76S for a couple of years. I beat that thing and it always keeps on working. It's supposed to be waterproof (and float too) but I've never had it in the water. With this unit you can also upload a variety of maps into it (topographical, street, etc) but I just use it the way it came out of the box. I've used other Garmin GPS products and found them to be easy to use and reliable.
One feature that I really like is the "man overboard" feature. With MoB you can create a waypoint instantly with by pressing the MoB button. It can be handy when you're riding new territory and need to mark a point quickly and easily while you continue to pilot the boat.
It's also got cutomizable screens so, for instance, if you wanted to display your speed, time of day and heading on your map screen you can. If you want to, you can keep the display backlit which is great at night. I'm not sure if all GPS units have a zoom in and out feature, but it's something that I use a lot. Now that I've used it, I wouldn't buy a unit that didn't have zoom.