The U.S. Navy's Marine Mammal Program started back in the '60s, and the dolphins and sea lions in it help defend harbors, retrieve sunken equipment and, most dangerously, identify mines for deactivation. By 2017, the Navy wants robots to do all that, instead.
The dolphins and sea lions the Navy uses really pull their weight: they're highly trained, and a team of veterinarians and handlers keep the animals primed and ready to be deployed anywhere in the world. The flip-side to that is that they're also expensive to train and maintain and, unlike a robot, when you lose one, you have to start all over again with a fresh animal. A robot arrives trained right out of the box, and you don't have to worry about its health in said box if you ship it abroad.