It's probably worth getting a cardiac checkup before dabbling too heavily in something like this. Immersion, and carbon dioxide buildup can do wacky things to heart rhythms.

If you have Long QT Syndrome or a genetic predisposition toward it, that can evolve into a "Torsades de pointes" arrythmia that may cause blackout or fatal ventricular fibrillation.

Makes me wonder if something like that precipitated Jay Moriarty's drowning.
https://www.youtube.com/watch?v=caEDDwCynps