I can't imagine there are too many waterproof universal remotes made. It would be a niche market for sure but surely someone has gone after the hot tub crowd.

Google did find this.
Weather resistant only, but it does say it is "completely sealed" It might survive brief immersions.

You might try the forums at RemoteCentral.com.
I found it a great place years ago when I was researching universal remotes. Hopefully it remains friendly and useful.

With great power comes Awesome irresponsibility.