Finis Swimsense watch. I asked for it after reading 10KMarathon Swim's review, not sure if Santa would really come through for me or not. Well I ended up opening it yesterday morning. This thing is awesome! I especially love how it helps me by confirming my total yards completed. This way if I lose track, I don't end up just doing penance, but just confirm on the watch. Only thing is, the display is difficult to read and so it causes me to pause for a second underwater to check the total, but I only check it about once every 2000 yards or so. It's right on.
The only time it messed up was when I was doing IM drills (one arm fly) and obviously it couldn't get a reading when I was doing my right arm. So I ended up "tricking" it in order to get it to count. I would do a one arm drill and then rotate to the other side every other stroke. Today's workout was:
16000 - 4 x (2000 free, 2000 pull)
2000 - 10 x 200's (odd IM drill, even free)
20,000 yards total in 5:13 (average 100 yd pace throughout the entire workout: 1:27)
Average stroke count per 25: 8
Here's a link to my data.
I assume the watch would be any good in open water, as I figure it relies on the configured pool distance and the sensing of a turn to really figure out how far you've gone. I read some reviews yesterday and was concerned about the battery life. But after spending over 5 hours in the water, it still had a half charge according to the battery icon on the display.