Now this is just awesome: DARPA is studying the construction of robot mini-frigates to track and follow submarines without any crew on board at all. The logic of it is that a small unmanned "platform" (which means, you know, a boat) can tail subs using noisy, un-stealthy active sonar -- pinging away happily all day long to maintain a fix on the target. This is difficult for present-day ships because of the danger that if a conflict develops, the sub's first target would be its stalker. But who cares if an unmanned ship gets sunk? Nobody has to write a letter of condolence to General Dynamics.
What makes this so awesome is that these military robots would be the real deal. Most combat "robots" are simple remote-control devices, bigger and tougher than your radio-control car from Radio Shack, but not necessarily any smarter. The human operator is the brains. But these robot ships would actually navigate (and follow the rules of maritime traffic) with only occasional contact with human command.
If it pans out, this could be huge. One obvious next step would be to dispense with crews on cargo ships. If a warship can tail an enemy sub, it shouldn't be too hard to program a robot freighter to sail from point A to point B. The only question is whether robotic systems are cheaper than overworked South Asian sailors (not yet -- but wait a couple of decades). How well robots would deal with pirates remains to be seen.
Another obvious application is to automate the submarines. Sauce for the goose, and all that. Of course, an ocean teeming with unmanned robot warships might actually increase the chance of conflict: people back on shore might well be willing to start a fight if they know that not only will none of their own sailors but none of the enemy as well would be harmed.
I can imagine a few decades down the line some desperate navy coming up with a wild scheme: put actual, living people aboard warships to deter enemies from shooting at them!