You could also set up a trap for allied navies, by sinking the dd and leaving your subs posted there.
If, by any chance, the german player was to be attacked by an english navy that would not dispose of a destroyer, they could trap them by submerging prior to the battle and coming back to attack the fleet on their turn, using planes togheter with the subs.
I don’t exactly remember the initial setup of the game and if uk disposes of a lot of destroyers, but in my opinion, this tactic could work at least once against inexperienced players.