NickiNav is a Wakeboard / Wakesurf app that offers a GPS Speedometer, and adjusts your media volume based on speed.
Lay your phone flat to measure Pitch and Roll angles to really optimize that surf wave. Start with 9.0deg Pitch and 2-5deg roll and make adjustments from there.
Purchase a NickiNav control module and let the app control the speed of your boat for you!