I've been looking up info about a GPS as well, and I found a post about using the GPS under water. The basic concept is that the GPS should retain its display even when it loses its signal.
The author of the post said the Garmin replaces its display screen with a "Signal Lost" message, while the Magellan doesn't. So the claim is that if you get a clear under water housing for it, attach it to a line, and ensure that it's bouyant. When you release the GPS it will surface and regain a signal. Then you can pull it back down and get your exact positioning.
As for the question of currents affecting the divers actual location, he says, "This has never been a problem because the housing ascends very, very rapidly. Even in the most sever current(including change of tide) I have not had a problem. I can often see the housing directly above me. Remember that this concept is being used in depths of only about 70fsw or less. *most often 50 fsw or less. At 50 ft....the housing hits the surface in about 3 seconds!"
I make no claims concerning any of this, but simply put this out for comments, since it seems that most on this thread (up to now,) prefer the Garmin.
Aside from the under water use of the Magellan, I thought the display being overwritten by a signal lost message was a little bothersome. Even on land, if I lost the signal, it would be nice to still be able to reference the last known location on the display.
Thoughts, comments?
Cheers,
Robin