So your bilge pump, or the water line into the lake, or something inside your swimming pool needs repair. Do you have to get it high and dry to fix it?
We tested three materials under water: a special epoxy called WetBond, regular epoxy and PL Premium polyurethane adhesive.
WetBond is a two part epoxy mix that is specifically formulated for application under water, and it does work. You can tell right away that something is right because it sticks right to the surface of about anything including PVC pipes right under water. This gives you the full strength of epoxy glues, under water.
The standard two part epoxy looked like it worked, but was very easy to pull off. Even while applying it didn't want to stick and had to be molded onto place. No for regular epoxy.
Then there is the easy to find PL Premium construction adhesive that you can find in most renovation centres. This is a polyurethane adhesive, which means that it sets by reaction to moisture in the air, not by any solvent evaporation or chemical reaction to a two part catalyst. It too wets right onto the surface under water, although not quite as easily as the WetBond so you need to be sure it is rubbed onto the surface and not just sitting there with a film of water between the adhesive and the pipe. Since water is what causes it to harden, it just loves it under water. Not having to mix it can help in under water applications, in fact you could dive in with a caulking gun. Because it is non toxic, you can even repair something in a fish tank, or the tank itself, without removing the fish. Despity using this in wet conditions for years, I have recently (2012) found a mention on its label that it is not for underwater use. There must be some long term problem that my fish haven't figured out yet. More seriously there seems to be a distinction made here that some glues can be used in an outdoor, and occasionally wet environment but not necessarially continuously under water. So continue to use the PL Premium outdoors, but either use the WetBond underwater, or dry off the surface and use Silicone sealant/caulking.
One tip to working under water is to clean off the surface to be glued with steel wool, using the water around to rinse it clean as well. This will take off any scum on the surface that would prevent a good bond. Then work the glue onto the surface, to push the water out from between the glue and the surface.