Meet Marlin: a wearable trainer for swimmers that gives them audio feedback with distance, speed, lap time, and navigation information. It uses bone conduction technology to help you hear. It uses GPS and motion sensors to monitor your performance in open water swims and pools respectively.
Marlin provides you with lap count, time, stroke rate, and swim time. The device is waterproof to 1.5 meters for 30 minutes. It connects to your smartphone via Bluetooth. You can find out more about this project on Kickstarter.