I wouldn't get too wrapped up in the argument that insects don't swim across the stream. You look at your wet fly and think it is "an insect." The fish doesn't. The fish sees something that is swimming, and is therefore alive (and is therefore edible) and what is more, is getting away. Drag on a dry fly will kill you. Drag on a wet fly makes it act a lot like a minnow. Trout eat minnows.