I completely agree with everything that's been said about getting a marine unit... however, if the budget doesn't allow that right now, all you have to do is set your Garmin's "home" destination to your launch point.
I have a Garmin C330 that I used in my car for a number of years... but I also used to use it as my "close offshore" GPS until I got my M56 marine unit GPS/Fishfinder. Like MC says, far offshore all it will show is a big blue screen, but once you get close to land it starts to show details again, and most important, when you tell it to go "home" it points you in the generally right direction. Get a Marine uinit and you'll be happy you did. Just don't forget that even the best GPS can fail. ALWAYS have a reliable compass on your boat even if you have a GPS.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God)
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.