OK, there are some really great ideas here!

Rolf: Gyro Gearloose, my hero! Could you explain to me how you would like the tack / gybe timing function to work? To be honest I can currently rely on the field to keep me on course...
For the leash I was thinking of a mega velcro strap. I'll be sure to add a waterproof speaker with a vol. control. (maybe tunable frequency in case my years at the disco start to catch up with me...)

Capt Cardiac, a swivel sounds cool. I could add a detector, with suitable logic so it doesn't flip on every wave, to automatically flip the display so I don't have to read upside down (maybe that would require software controlled buttons too). Still, good food for thought.

Tornado, that sounds good do you have pictures?
David Ingram and Capt Cardiac. I'd bet on e-paper technology to give us wearable ultra light screens "Real Soon Now" (c)(tm). Just imagine the drama if you forget to change the batteries in your main! (hope they make them solar powered).

Thanks everyone, this project is really starting to materialize in my head now. I won't be doing any GPS interfacing because the required modules suck too many batteries for my tastes. Maybe on a next version in the future though...