The water dunk helps blow off some of the carbon and oxidization that inevitably occurs during the heating process, but the main purpose of quenching copper is to quickly cool it, as copper doesn't harden when quenched (unlike some other materials). It's not required, but it results in a better finished product, and less chance of a burn.
Same goes for the sanding, it just knocks down the high spots from the previous installation, giving you a better surface to seal with. If the washer has ridges that catch your fingernail, that is a good candidate for a quick post-anneal sanding.