Just wear it underneath. I did (when I used a chest strap)

Yeah I did that but blew out the Garmin Heart Rate Strap. Don't think it's waterproof. At least the one I have. I have the soft strap. Maybe I need the hard plastic? Which one did you use?

http://g.factoryoutletstore.com/cat/29449/Garmin-Fitness-Heart-Rate-Monitors.html

I've used both. Never had issues

I'm 99% sure all the Garmin straps are waterproof.
They'd be pretty ridiculous for triathlon otherwise.
I've used both the older premium strap and the newer tri strap in the water regularly. I may have used the old classic hard strap in the water too but can't remember for sure. There should be no problem. Could be a coincidence yours died during/after a swim or maybe there was a defect or it hadn't been closed properly after a battery replacement?

Been using the Garmin heart straps under tri tops for years without any issues. Hard plastic with the 310 and right through all the variations off the recent soft strap. Never had any problems with the sensor modules, ever. They are waterproof.

As suggested above, you might have just been unlucky or maybe not put it back together properly after a battery swap.

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Trust me I’m a doctor!
Well, I have a PhD :-)

Ditto here. I've never had an issue with the HRM under a tri-top. I have a Garmin soft strap and it works like a charm. It has the added benefit of doing an awesome job of holding ice chips in place while running :-).