You can actually do it wet but I do shove a rectangular piece of foam in the skimmer opening then suck the skimmer dry with a shop-vac. I leave that foam in place over the winter bacause I don't drain below the skimmer and it isolates the ice block in the pool from the one in the skimmer. The foam doesn't form a good seal but good enough to let you suck the water out. As for curing, silicone doesn't want to stick to wet surfaces but once in place it will cure under water.
Al