Here's a tip I figured out for using the Tempo Trainer in open water. If you enclose it in a Ziploc sandwich bag, it will float. Then just stick it under your cap like normal.
I dropped my Tempo Trainer in the lake a few weeks ago, and it was GONE. They don't sink very fast, but I couldn't see it in the dark water. I was doing speed drills with my Masters team, and my tired little hands just didn't grip it tightly enough when I was adjusting it. D'OH!
(For more on this fascinating topic, check out this video: http://www.youtube.com/watch?v=WfUSyP3gqAw