@Cernel Sorry for answering only by now. It has been a quick question but the answer took me some time, that I did not always have during the last days.
Actually I would like to pass the answer to your question to @Krieghund here.
LHTR state on page 29: “Defending submarines may withdraw by submerging after any round of combat.”
But this is explicitly restricted on page 16: “If the attacker has not withdrawn all units, defending submarines may submerge.”
In other words: In case there is nothing left to withdraw from a defending submarine cannot submerge.
OOB rules unfortunately lack this clear statement.
A hint could be found on page 32. Here it reads: “A submarine may submerge in combat after the attacker and defender have fired, regardless of what other units do.”
Now the words “may submerge in combat” could give a hint on how to deal with a defending sub when the attacker has retreated/submerged all of his units, as we could consider combat being over then: no more combat -> no more submerging.
But again, this is not explicitly stated anywhere, neither could I find any past clarification about a submerging defending submarine in the given scenario, so let’s see what @Krieghund might have to add here.
Out of curiosity @Cernel, which scenario do you have in mind where you as defender would want to submerge a defending submarine, although the attacker retreated/submerged everything?