I have a Magellan Merridian Marine but it's the only one I have exp with so don't know about the others. It was a little bigger then the others and had a marine database. Look into one that has extra space available to download more info, I need another memory card to download any extra stuff.
Also check with Vanns.com before you buy, mine was about $100 cheaper from there compared to anything around town.
Check for good prices and service plus a free Tshirt at www.gpsdiscount.com
I've had good luck buying from them, I have a Garmin GPS and a Garmin fishfinder.
Definately get the Blue chart if you ever leave familiar waters and go on the road.
I have the Magellan Sportrak Pro Marine and have been very happy with it. It is my third gps and is by far the easiest to use that I have had. I'm guessing there are many good units on the market right now. I would test drive as many as possible.