There are some US AI subs roaming about, not co-ordinated with headquarters at this time, but their torpedoes are visible, as are their explosions. You would also be able to hear them on sonar, see them on radar and sight. Nothing AI submerged (at this time). I don't think there are any placed in that area though, and they would be on the surface, with the IJN running away from them. The AI subs seldom get to use their torps, except on the IJN AI subs when a certain USN sub skipper isn't paying attention
What was the weather like Immelmann, and were they already low in the water? There may have been an earlier carrier plane attack, or an AI sub attack on them that caused damage, but bad spawns can also set the ships too low at times.
