Like the Americans (e.q. Elco 80), the Japanese had a number of fast attack crafts, which were used to monitor reefs and atolls of the Pacific islands.
Ingame, fast attack crafts are surface combatants whose torpedoe can be dangerous to much larger ships. However you shouldn't attack frontally because of the barely existing armor, but take an advantage of the speed to emerge from an unexpected angle, fire torpedoes and soon to disappear.
With the mines, you can make access to bay more difficult to the enemy - but also to own team - or even completely deny it. Since the mines are clearly visible, anyone rarely get caught on them.