From the canonical source of all questions fitness watch related: http://www.dcrainmaker.com/2013/04/suunto-ambit2-review.html

> Note that the ANT and ANT+ transmission protocols that the Ambit2 utilizes to transmit heart rate don’t transmit well underwater (about 1-2”), thus, you won’t get any heart rate data.

I believe Polar's ultrasonic sound transmission is still the only one which works reliably underwater.