The best value right now in GPS
units for both marine and automotive use are probably Garmin
's 276/378/478 models. The 276 is available with both BlueChart and CitySelect maps on CD, where as the 378 and 478 have the US-automotive maps and either the inland (378) or coastal (478) BlueChart G2 marine charts
All three units IIRC can be expanded with Garmin
Memory Cards, but the cards are proprietary.
I have the 276 unit, and use it as a backup on my boat as well as the primary unit for my truck.
These are not handheld
units, like the 60xx or 76xx units, but are rather a bit larger, with voice prompting for the automotive directions.
You can also use either the 60__ or 76__ units from Garmin
. If the unit has a X in the name, it takes flash memory cards (mini SD I think) otherwise it is limited to the base memory of the unit. BTW, I used the 76CS for many years for my fishing trips...and it lives in the ditch bag on my boat.
60c/60cs/60cx are a bit smaller than the 76c/76cs/76cx units, but IMO the controls aren't as good and the 76 series has the advantage of floating, rather than sinking...so if you drop them, you're less likely to lose the unit.
Worst Marine has had the 276 units on sale recently...so might be worth looking at.