Marine units need to be more rugged to stand shock and waterpoof. My marine unit (Garmin 276C) has features that my daughter's Nuvi unit does not have.
I got an old streetpilot off e-bay that works just fine for me, does everything I need it to do. Probably depends on where one is boating though. I'm boat on the largest lake in Kansas which in my home state of Arkansas would constitute as a very large pond or a small city lake.
Some 'automotive' GPS's can have marine maps loaded, others can be hacked. Here is the Nuvi 500 which is pretty cheap. And how to make it work with charts/boat mode http://www.last-outpost.com/~malakai/garmin/index.html.
It's the same hardware, but the software is different
