I have a garmin e-trex handheld. It comes ready to go. Theonly option which I have never used is, i think you can download more detailed maps. If you go on garmin's website, I'm sure they can explain it better that i can. BTW I am selling mine. I don't have the original box but it works fine and doesn't have a scratch on it. Looking to get $100.
Fishead...You need to have a good mounting bracket and good connection to the adapter or if your using batteries make sure it doesnt get bounced around too much or it will cut off when you take a big wave....its hard for someone to hold it if your on a smaller boat..