As an avid user of two GPS units, I second John's take. I have the NAVAIDS CD for my newer Garmin unit, and on my local lake, they put a marina two miles from where it really is, and on the wrong side of the lake. Unless you know the waters well, don't leave the dock without a good set of paper charts (my 1978 Charts of the Chesapeake WERE good!). I'd say GPS is good for gross navigation (pointing you in the direction of a town 10 miles away) giving you a good fix on position, speed over ground, distances to go, etc. And over the years I've developed a trust in them, as even my small handheld unit will consistently point me to within a few feet of my driveway when in my truck. So I also trust them to help navigate over open water as a supplement to a compass that is 4 degrees magnetic off from the heading the GPS unit gives me, while maybe sliding another 10 degrees off course due to leeway, and further complicated by the affect of tides and/or current. It keeps pointing you in the right direction and in that way, will save you from making gross errors in navigation. But even with that type of helper, you should make frequent plots of your position and when you get where you are going, or in a tight spot, get out the paper charts and get yourself oriented. And about the sickest feeling you can have with a GPS unit is to be out of sight of land and have the batteries go dead. Howard PS. If you want to test your unit to see how accurate it is, set it outside in full view of the sky for about an hour. Then switch to the screen that plots a track of your movement. Set it to the smallest scale available (maybe .1 or .2 miles). A plot of the error should be on your screen. The unit hasn't moved, so any apparent movement on the screen since your test began is the error of the unit. Then walk up and down the street, or around the block. You should be able to draw a path around your error like you were using an etch-a-sketch. That's how far off your unit is....at least for that position and at that moment in time.