There seem to be two different discussions going on here. The original "Bent his breast" and its meaning, and "Smote his breast" . I have always understood "bent his breast" to mean something like "(jumped in and) started to swim". Certainly the contexts quoted so far (Musgrave's page bent his breast and swam, Goldsmith's character bent his breast against the broad wave and the ship's boy on the Golden Vanity bent upon his breast and away swam he..) all concur with that. I don't see how you can stand and lean forward (the other suggestion so far) while you are swimming!
For the other debate, to smite upon one's breast, I agree with Lighter To "smite" your breast means to strike it with the hand or fist, usually as a gesture of shock, grief, etc.