@general-LD Rulebook says:
"Submersible: A submarine has the option of submerging. It can do this anytime it would otherwise make a Surprise Strike (see below). The decision is made before any dice are rolled by either side (the attacker decides first) and takes effect immediately. When a submarine submerges, it’s immediately removed from the battle board and placed back on the map. As a result, it can no longer fire or take hits in that combat. "