Designed specifically with sailors in mind, the Velocitek S10 would be handy for people who own/run dive boats, as well. Featuring 2 over-sized buttons and 2 large digital read-outs, the S10 is a waterproof, "high-accuracy GPS Speedometer / VMG Sensor with a 10 hour GPS data logger." Designed to work seamlessly with both Google Earth and GPS Action Replay, the 0.45-pound unit allows users to download, view, and analyze the GPS data recorded while on the water.

Capable of recording latitude, longitude, time, speed and heading, the tiny unit -- fabricated from bomb-proof polycarbonate -- is waterproof to 75 feet. Although I don't think you could dive with it to that depth and expect it to record data, you could easily strap it to a float and tow it with you on a shallow dive, or *maybe* stick it in your pocket as you snorkeled. Unfortunately, the $299 unit is XP-compatible only.

[Via Waterlog]