You can use any silicone that doesn't have anti-microbial additives. GE Silicone I is a good one, but there are other cheaper generic brands. Just stay away from the ones that say they contain mold and mildew inhibitors.
Silicone will not set up in the water. And it also takes 24 hours to cure. You'll have to do it while everything is dry, then leave them be.
Silicone wont really stick to a wet surface. I am also not sure if it would cure under water. Also, silicone releases fumes while it is curing. If it does cure under water, those fumes would be soaked up in the water and most likely harm or even kill the things living in your tank.
As for the type of silicone that is safe to use in our aquariums, anything that is 100% silicone and DOESNT have any anti microbial or anti fungal chemicals added to it. I use the loctite brand most often just because it says aqquarium safe on the back package. ALthough a cheap store brand would work too.