Garmin Swim 2 will measure your heart rate under water
Garmin Swim 2 | Outside The Asylum Diving & Travel
Garmin Swim 2 review
Garmin Swim™ 2 | Swimming Watch
Amazon.com: Garmin Swim 2, GPS Swimming Smartwatch for Pool and Open Water, Underwater Heart Rate, Records Distance, Pace, Stroke Count and Type, Slate Gray : Electronics