A seagoing BOLO?
Would be really cool. Fire control could be automated, so the ship would automatically fight back if threatened or attacked. No crew to risk, no chance of anyone taken hostage. No need for food or water or bunks, so more room for fuel, ammunition, armor, and so on. In addition to hunting subs, ships like this could be used to constantly monitor suspected pirate bases, terror camps located near the coast, and so on. In a way this would be a very good way to save on limited manpower. Since they could stay at sea in large numbers they could also act as a blockade force (off the Somalian coast perhaps?) as required.
All in all, a very good idea.