Dangerroom reports on a new technology being developed for the Navy, the Anti-Submarine Warfare Continuous Trail Unmanned Vehicle, or ACTUV,. This unmanned surface vessel is intended to dog potentially hostile subs during that awkward period when tensions are high, but before the first shots are fired. The idea is that once the sub is located, one of these unmanned (and at least for now, unarmed) surface vessels will be assigned to trail it using active sonar and other sensors. This should cost less than maintaining a continuous track using Maritime Patrol Aircraft, and would allow manned vessels to avoid coming within range of the sub. If shots are fired, presumably the ACTUV would be the first to go, but it would be a minor loss, and allow the manned vessels to avoid being surprised.
The technology may also have some implications for the Coast Guard. We might see a smaller version of this launched from a cutter to augment the cutter’s radar picture. The technology for this requires developing an artificial intelligence capable of applying the rules of the road–essentially a computer OOD. Some day the Coast Guard may be asked to approve fully autonomous merchant vessels plying the trade routes with no one aboard.
Chuck Hill's CG Blog 
First successful voyage: http://www.navaldrones.com/ACTUV.html#Surrogate
An update, looks like it is going to happen, although the report gets in wrong in talking about tracking quiet conventional subs only in US waters. http://www.dailymail.co.uk/sciencetech/article-3011630/The-Navy-s-ghost-hunter-hit-water-Robo-boats-set-track-silent-enemy-submarines-months-time.html
Looks like the prototype will launch this fall: http://www.nextgov.com/defense/2015/04/us-military-deploy-robotic-ghost-ships-fall/109219/
Looks like this is actually happening with sea trials coming next year. http://science.dodlive.mil/2015/11/09/actuv-sea-trials-set-for-early-2016/
Also interesting to see the daily operating cost of a destroyer, $700,000.
Prototype to be christened in April. http://thediplomat.com/2016/02/worlds-largest-anti-submarine-robot-ship-ready-for-sea-trials-in-april/
Here we have a photo. http://foxtrotalpha.jalopnik.com/exclusive-this-is-the-first-known-image-of-darpas-subm-1759205822
ACTUV is in the water and in trials. http://www.marinelog.com/index.php?option=com_k2&view=item&id=10781:video-darpa-unveils-sub-hunting-super-drone&Itemid=231
trials completed. http://www.navyrecognition.com/index.php/news/defence-news/2016/july-2016-navy-naval-forces-defense-industry-technology-maritime-security-global-news/4245-leidos-completes-initial-performance-tests-of-highly-autonomous-unmanned-surface-vessel.html
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Looks like the Brits are planning something similar. https://www.c4isrnet.com/unmanned/2017/09/19/now-rolls-royce-is-building-robot-ships/
Apparently the Chinese are also working on their version of an autonomous long endurance surface vessel. This from the German Navy blog “Marine Forum.”
“18 September, CHINA, China Aerospace Science and Technology Corporation (CASC) has revealed plans for a new 30-m autonomous multirole surface vessel (USV) designed specifically for high-end naval operations (ASW, ASUW) … D3000 to follow trimaran design with hybrid-electric propulsion … capable of 90 days of autonomous operations.”
And there is this, again from “Marine Forum”:
“19 September, ISRAEL, New „Seagull“ unmanned surface vessel (USV) in a live demonstration off Haifa successfully carried out an anti-submarine warfare (ASW) mission, using onboard dipping sonar.”
This one looks a lot more modest, but the Russians are in the game too. https://www.navyrecognition.com/index.php/news/defence-news/2017/september-2017-navy-naval-forces-defense-industry-technology-maritime-security-global-news/5575-kronstadt-completes-first-tests-of-unmanned-surface-vessel.html