I have just bought the Rhythm+ HRM. You can strap it to your forearm or upper arm and so far, it is working well. I don't know that the Velcro-like fastening will last long as it is not very sticky...but so far, so good.

I also looked at the Mio ones but don't think I have room on one wrist for both HRM and Garmin (although it might even be able to capture swim HR if it is waterproof)!