Actually, there are two versions and one purports to be waterproof but here's their own take on how well it works or might work depending on water depth (not very deep)
Amazon.com: Customer Questions & Answers
"It still transmits under water but the water attenuates the signal quite a lot. We have actually taken some data on this and made some calculations. What we found is that 6 inches of submersion would cut the maximum range of operation to about 1/3 of a mile, 18 inches of submersion would get you down to about 400 feet of range and at 6 feet of submersion you lose all contact, even from directly above the tag. With these kinds of losses there is little that can be done to improve the situation, even if the transmitter was 100 times the power it would not even double the depth at which the system would operate. "
I keep toying with the idea but so far have not sprung for either version. I can see losing it in a snowy field - amazing how snow can conceal the crater!!!