Lowrance used to make automotive gps's that could take a secure digital card which had the entire coastal US. The card was only about $100. I still use mine as a backup, but it's not waterproof. Garmin's Nuvi 500 does both, with the addition of their blue chart sd's. I read that Garmin has also come out with an inexpensive coastal sd card. And I think I read that some Magellan gps's will do both but their charts were expensive.
One problem is that automotive gps's do "turn by turn" routing, showing each street to turn on. Marine gps's use "point to point" routing, which just goes from waypoint to waypoint. If the gps isn't programmed to be able to do both, you're stuck with one or the other.
Also, in many cases, the charts are expensive and limited to one manufacturer's hardware. It may be just as cheap to buy a the unit and charts as a package.
Here's an alternative: last month, I cruised along the Gulf Coast using a netbook that has the latest free Sea Clear charts. It connected to a bluetooth gps sensor, so the netbook could stay down below, nice and dry. It worked very well. Compared to even the automotive gps's, the display was huge so it was very easy to read when we got into shallow areas.