http://www.platysens.com/marlin/
From the website:
Tired of looking at your watch or the pool’s clock for every lap? Marlin provides realtime audio feedback to swimmers. Important parameters such as lap time, stroke rate and navigation information* is provided to swimmers without disrupting the swimmers' motion. Whether you are swimming in open water* or in the pool, Marlin can tell you know how well you are doing so you can focus on your swim.
From the website:
Tired of looking at your watch or the pool’s clock for every lap? Marlin provides realtime audio feedback to swimmers. Important parameters such as lap time, stroke rate and navigation information* is provided to swimmers without disrupting the swimmers' motion. Whether you are swimming in open water* or in the pool, Marlin can tell you know how well you are doing so you can focus on your swim.