The U.S. Navy has expanded the number of jobs, its force of about a hundred trained sea mammals (dolphins, sea lions and seals), can carry out. The animals have now been trained to carry a tow line down to an underwater object, so that it can be hauled up, or simply into view. Some of the mammals are also trained to patrol an area, equipped with a harness containing a camera.
Two years ago, the navy sent 30 trained dolphins and sea lions to help guard a submarine base in Puget Sound (near Seattle, Washington) against hostile swimmers. The dolphins are trained to either drop beacons, if they spot a swimmer, or slip a cuff around a swimmers leg. The cuff is attached to a rope, and this allows the dolphins handler to reel in the swimmer.
In 2003, some of these sea mammals were sent to the Persian Gulf to guard against hostile swimmers getting near coalition ships or port facilities with bombs. Called the "Mk 6 anti-swimmer dolphin system", these dolphins normally worked with Explosive Ordnance Disposal units. This is because the dolphins are mainly trained to search for underwater explosives and mines, using their natural sonar ability.
About as intelligent as dogs, each sea mammal bonds with its handler. Without that bond, the sea mammals could just wander off and not come back when turned loose to work. Occasionally a sea mammal will disappear for a few days, particularly when there are wild members of the same species in the area. But dolphins and sea lions are pack animals, and it is difficult for a domesticated sea mammal to be accepted into a group living in the wild. The sea mammals tend to bond with their handler. It takes about 18 months to train a new animal, and most can continue to serve 10-20 years, before being retired.
The U.S. Navy has been training the sea mammals since the 1960s, and has found that they are as trainable as dogs, but live twice as long. When too old to work, dolphins and sea lions are retired to a facility which feeds them, looks after their health and lets them out for swims in the open water. These are domesticated animals, and prefer the company of humans. Because they work under water, these animals don't get a lot of attention in the media.
As successful as the sea mammals have been, they are being replaced by underwater robots. As these torpedo size machines get cheaper and more capable, the sea mammal program will be phased out.