Tankers have DM. So it depends on where you hit them.
In online ships are bugged though atm I think. Just not sinking if hit by a player, but can be sunk by bots I think.
E.g. Not sank after direct hit plus close underwater explosions on Repka #3 server today.
https://lh5.googleusercontent.com/-d...5-26_00003.jpg
https://lh3.googleusercontent.com/-p...5-26_00004.jpg