With underwater explosion in the direct proximity you do not need that much, especially if you will use a charge with explosively formed penetrator. 16 meters o

With underwater explosion in the direct proximity you do not need that much, especially if you will use a charge with explosively formed penetrator. 16 meters of water pressure will quickly exploit even a small hole. My guess, it's small enough to be deliverable by a remotely controlled boat like those used by Ukraine to attack the Russian fleet, naval bases and the Crimean bridge.