I've done it with my cousin once. Its was pretty cool, great way to get learn GPS skills. I'm going to try a "marine geocache" out on the Chesapeake Bay this coming summer. I'll attach a cache to several buoys and anchor them. Have my friends try to locate them in Boston Whalers and wave runners.
Wow this is old. I consider myself a Geocaching fanatic. I use Cachemate to store all my intended victims along with previous logs of the sites and clues. I use Terraserver to view imagery of my prey. I use an iQue 3600 with MapSource City Select v6 to route me to the location or find the nearest parking lot. I use a Garmin Vista with Topo for the actual hunt. I have my laptop with GSM card to log the visit, get up to date info, or just be first to log the visit. Yeah I got a problem.