A gps will work anywhere. Now having said that you will not be able to "map out a trip with it" you will see where you are in relation to where you are on the lake. Check with tom tom to see if you can use it to backtrace your route though. I bought a hummingbird fishfinder with gps and it has a chip that I bought from navonics for my Canadian trips. Won't leave shore without it up there. Way too easy to get lost.
If you mark the ramp as a waypoint it can show you where that point is relative to where you are at any given time but if on a very large lake it will likely show a blank screen with the car image pointing in the direction you are going. Don't listen for turn-by-turn directions to the next bar or fishing hole though (especially if you intend to go there by water). Sorry but I couldn't resist that. Auto based GPS have limited waypoint storage so the number of spots you can save is limited. Not a practical device for that. A cheap handheld GPS would be much more practical.
When I'm on the lake I use my Garmin 205 to take me back to the landing. I don't the lake that well being a new boater. It will show where you are at on the lake but that's about it. With the nuvi you can spend the bucks and buy the lake maps. When I hit the lake this year I'm going to try it in pedestrian mode and see what it'll do. Cut off the voice. It'll try to get you back on the road which is annoying.
I have a etrex handheld gps that works off 2AA cells and I can wire into my fishfinder. It gives me speed, waypoints, heading, distance to and from a waypoint. You can also enter lat and long to set up waypoints at the house. The best thing about it was it only cost $79!
