Great for long summer days at the beach, or boating/surfing, when kids are in and out of the water and hands are often tied up holding lines or maintaining balance. At least, that's my experience with Li. Even when she's in the water when we're boating/swimming, we often find that she's facing another direction, using her arms, or otherwise unable to sign as easily as she can hear/speak and so we tend to waterproof the CIs for those times. If you think about how much time a kid is actually submerged when around the water, it's pretty low. We then really bag them up, tie them to us or the boat and tuck them away when she's doing things that involve being fully submerged. If she were on her own, I could see them being tossed onto a towel when she runs into the water -- easy to lose that way. I'd rather they be fully submersible and stay with her even if she's not using them under water.
But ... we don't have those