Hi,

New to the site, and was here reading last night and thought I'd give my 2 cents

Silicone should not be used underwater what so ever. Silicone contains talc which allows it to absorb water, and it gets brittle over time and looses it's grip. Ever notice when you put a clear bead of silicone in your shower that it's nice and clear, then a few years later its yellowed, and not sealing very well.

I work on lg. privatly owned boats for a living and silicone has never been a good thing on boats.

The best products to use for gluing/sealing anything that has to do with water is Polyurathane Marine Sealants/Adhesives.

3M makes a couple good ones 4200 (fast drying) 5200 (dries very slow but is extremely strong)
Sikaflex 291 is a good all around sealant adhesive.

I come from the Koi ponding forums where I have been preaching the use of these sealants for a long time. I made my own 4000gal pond out of fiberglass and have come here to see if anybody has taken on building a pool as I am in the planning stages of starting my own pool (beach entry, my own pebble finish) out of glass. But I haven't found anything yet.