I used to swim 1 x/week...and used to use my Polar S-510 HRM in the pool. I never had any problems, but I didn't hit the buttons when in the water...I made sure to be up, out of the water, and my hands free of dripping water.
I don't think it's the strap that's a problem, it's the buttons on the wrist unit. The strap gets sweaty and wet all the time when running, cycling, etc, so I don't think a pool would matter much.
2007 Seven ID8 - Bontrager InForm
2003 Klein Palomino - Terry Firefly (?)
2010 Seven Cafe Racer - Bontrager InForm
2008 Cervelo P2C - Adamo Prologue Saddle