“Diesel boats are very quiet under water running on batteries.”
And far superior to what they used to be. It it a common mistake to assume that after nuclear boats were developed that diesel boats simply stagnated.
Nuclear boats carry batteries in the event that the reactor scrams thereby leaving the it helpless.
The Germans has developed fuel cell powered "littoral" boats that are as quiet as a tomb. The Swedish design uses a Sterling engine fueled by diesel fuel and liquid oxygen which is also dead quiet.
Regards,
GtG