I don't know about using it underwater, Polyurathane gives off some nasty's while curing. It would harden underwater for sure, but not sure how toxic it actualy would be while curing. I've always waited 12+ hours to put the stuff in the tank(They say 2 is cure time, i give extra). Gorilla glue is pretty much the same as the PL Premium construction adhesive, same base chemicals, slightly different mix.
Edit:
Here's the MSDS for the glue, not very safe in liquid form. I'd keep it out of the tank wet.
http://www.gorillaglue.com/Portals/0...%20Jan2010.pdf