Garmin HRM strap

I don't see why it would be a problem.

Yes, not an issue at all. Just make sure it's comfortable.

I've worn mine during the swim portion for a few races , never an issue

Thanks, all!

Actually, that's probably a better option than wearing it above your tri gear.  It will keep the strap from coming off during the swim.  Also, there's been some comments within various forums that dri-fit material between the strap and transmitter tends to make the heart rate transmitter go haywire at times (assuming that's the type of material you'll be wearing).  And I don't think having your clothing above the transmitter will have an adverse effect.  

If you find you don't like to swim in it...try this:

Similar to how you slip on your race belt (keep it fastened, put it on like shorts) slip on the HRM strap.  Takes all of 3 seconds if you practice it before the race.  Try it out, might be worth the 3 seconds for the extra comfort.

I did it for my HIM and it works just fine.