Here is a smart wearable device that provides swimmers with audio feedback underwater to help them stay on the right track. Marlin uses bone conduction technology to help you hear. It has GPS for open waters and motion sensors for pool tracking. The device also features Bluetooth.
Marlin provides you with lap count, lap time, stroke rate, and total swim time. It can also help you navigate in open water. The project is getting support on Kickstarter.