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.