since we're brainstorming..
Why not build the watch into your sunglasses, with a "heads up display" in the lower corner showing countdown timer. The "brains" of the unit could be similar to a cell phone somewhere in the boat, and connect via bluetooth (or some other technology) to the glasses.
Vibrating feature would be nice, or some kind of earpiece off the glasses.
Once the countdown is complete, the display could be user-programed to switch display to VMG, COG, heading to next waypoint, etc. just like a handheld GPS.
If it's not integrated into the sunglasses themselves, maybe it could be a "flip up" type semi-transparent display that goes over (or under) an ordinary or perscription lens.