Have a look at a Garmin GPS V for road and marine use.
I have had a GPS III for ages and have used it on RIBS, yachts and in my Landrover for Off Roading and even in aircraft. I use it with Memory Map Navigator to plan and review routes/tracks.
The V has good route planning for road use now that Garmin have sorted the "Turn Left" on roundabouts to "Left - Right-Straight On" at roundabouts problem.
Excelent battery life, totally waterproof and bullet proof. Comes with Mapsource software, mounting bracket and cabling.
Down sides are - Pricey with small B/W screen and it will not float. (Waterproof bit only usefull if you dive!)