Red, I'll add to the info "Secret Squirrel" gave ya. The PL is much better than silicone, I wouldn't use silicone in or around my pond if my life depended on it. The best stuff is a product called Sikaflex 291 it usually is available at Marine supply stores i.e., West Marine. Any marine grade Polyurathane caulk is the best for the job of sealing and gluing stuff in ponds.
If you don't have a boat yard or boat supply place close by, you can order it on-line from
www.westmarine.com the item number is 174508 for the Black Sikaflex, another good one is 3M 4200 item number 593139. They are both $17.99 a tube, but well worth the $$$$$.
I have used PL around my pond but not underwater so I can't say yay, or nay for underwater use, as I always have a tube of Marine grade caulk around here somewhere.