Unmanned doesnt means free from human control, just imagine UAV but underwater. With all electrical instrumen installed but you can controll it in somewhere else.You mean UUV?
No need for super advanced AI.
Install acoustic library on it, let it wander on designated area, keep submerged mostly only go to periscope depth for comm and to recalculate position, sunk any target that match the selected acoustic signature.
The main problem is, the radio wave communications isn't good enough to travel via water