Zander, pardon my ignorance but what good would a barometer do you once out on the water? Wireless sounds nice, its certainly doable (but adds the complexity of an extra receiver and its housing). I could look into induction charging to keep the system water tight (much like an electric toothbrush).

Tony, I think a GPS logger is probably best left to a dedicated GPS unit. Data captures could be synced later in software (if it turns out I actually want to capture data!)

I think for this project I'd like to focus on doing just one task, and doing it really well. (can you tell I like Linux?)

I agree Jake, I always forget exactly how many seconds late I was in reacting to the last signal... re-syncing at the next signal (or maybe even averaging) is an absolute must for me.