Did you know being immersed in water also slows down your heart rate? It has something to do, presumably, with being inside a bag of water for 9 months prior to being born.
You can slow down your heartrate by relaxing, controlling your breath etc. Yoga and other things like that can help.
Freedivers make use of this when holding their breath. I've read somewhere that there are freedivers whose heartrates slow down to less than 10 bpm when in cold water. Sorry I can't be more specific about that with a link or names. Anyway ... apparently the slow heart rate helps you to hold your breath for over 6 minutes!