any sports tracking app will do the job.
The most popular ones are Sports Tracker
I use SportyPal on my bike and also to record my kite downwinders and it's really interesting to see your track afterwards and check the speed on each tack (when you finish your session the program uploads the data to the website).
Here is a short downwinder I did in April this year: http://sportypal.com/Workouts/Details/2113624
and here's what a wave session looks like: http://sportypal.com/Workouts/Details/2139634
If i was into racing i would definitely track all my sessions. And don't worry about the phone. Just purchase a good watertight pouch and put it inside your wetsuit.