Hi,
have to ask if your HRM (Garmin) belt is linked to an old version of a watch, that still has an analougue signal? The reason why I m asking, is that only HRM watches that come with a HR Belt which have an analogue signal can travel through water. Most new watches tend to be with a digital signal, which can not travel through the water, so the watch will not pick up your heart rate from the HRM belt.
But either way no damage should happen to the belt when wearing it in the water, I have found no problem with the older version or the newer versions.