I was gifted a GPS watch that is waterproof last Xmas and decided to try it out. Apparently it has an pool mode and OW mode. My daughter is taking a diving class at the nearby pool and as she was in class, I decided to try the watch out.
I really liked the watch and after 2 laps, I did not notice it anymore on my wrist. It keeps a good measurement as long as you KEEP MOVING. I noticed a few times during breaks that it would go up by 25 yards even though I was on the starting wall. The watch ended up showing 50 yards more than I really swam. Not a big deal but it does help to keep you limited in your rests.
I just did a lot of 50's starting at the watch underwater and the most I did was a 500