Actually the GPS part is the same. There are only a few chip makers out there. A GPS is mostly a single
chip. A good car GPS is actually harder to build and they don't really have any on the market. They don't in practice work
well downtown because GPS is a line of sight signal. The signal bounces around and screws up the satellite
signal timing. When you lose GPS lock it takes a while to reacquire the lock. A boat GPS is already far more accurate given the lack of obstructions. The NMEA
out isn't a huge problem.
The display needs are actually more demanding. The super bright displays do cost more.The issue of brightness is significant as the lighting
level in a car is less than in a boat. The design of the case need only be water
resistant to a far less degree than for marine use.
Car GPS systems have other problems. Mostly the maps are pretty poor. Address accuracy is also poor and often incorrect. Lots of missing streets. There really isn't a good program for keeping them up to date either. The single
chip prices are way down now so in car GPS devices are easy to build cheap
. It does not mean they function all that great compared to what you get on the water