This is probably the result of a very "fractured" translation of the original Italian.
The depth bombs that were dropped exploded under the submarine and caused considerable damage. They also started a fire within the boat. This fire forced the crew to flood the ammunition storage area (locker) within the boat to prevent an explosion.
(I assume the ammunition was for the deck gun.)
There is another write-up of the incident here, but again the English translation can cause confusion.
http://www.regiamarina.net/detail_te...4&lid=1&cid=44
Just a note about the original B-25 incident, the navy was also operating B-25s (Denoted as PBJ-1H, PBJ-1J, etc. based on the B-25 model). So it is possible the "B-25" could have been a Navy or Marine plane instead of an AAF plane (if the incident actually happened).