Well, grpweld, you're starting to run into the fundamental questions here, and there is some online research you can do to help you resolve this. I'll see what I can do to help guide your research. I'm sure there will be other opinions, but here's my experience: 1) Hardware for applications (cars or boats) is specific to the vehicle. EG: a good car/truck/road unit will not be usable in the marine environment...a good marine unit would not be usable in the road environment.
Boaters like big screens, large buttons, waterproof construction, bright displays, and simplicity. Imagine what you'd like to have on the pitching deck of a sailboat approaching a harbor at night, and you'll see what's important.
Drivers like bright screens, lots of road information, data regarding restaurants, traffic information, simple buttons, and a screen which adapts to conditions.
There are units which attempt to do both jobs well (Garmin 376c as an example). The problem is that road-oriented units have road-type data loaded into the unit, and marine units usually have no data loaded...you have to buy what you want from the manufacturer or a third-party.
You, grpweld, have to figure out what you want. If you're not going offshore or long distances on your boat, then a crossover unit (road+marine) will probably do the job. However, I can tell you from hard experience that trying to read a small-screen GPS (Garmin 76CS) while the deck is pitching, the wind is blowing 30K, and you're sailing single-handed is not a great experience.
Now, it's time for you to do some research, and learn something about the fundamentals here.