So submerging is not a retreat. Interstesting. After reviewing the rulebook, it’s considered during step 2 the suprise strike. Which technically means by definition that submerging is a form of attack (Either roll dice or submerge). Because it’s during step 6 that you decide to attack or retreat, and then as you repeat step 2, since you have already decided to attack - you can submerge.
What I like about how the rules are written to, is that EACH sub gets to decide whether it fights or submerges. It’s not decided on a grand scale. You can submerge one, and fight/retreat/ with the others. This has come into play during larger naval battles for me.