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.
So, by 2017, this will be the Navy's new dolphin:
That's the Knifefish by Bluefin Robotics, which can operate continuously underwater for up to 16 hours — a dolphin's stay under the sea is measured in minutes before one needs to breathe. Like a dolphin, however, the Knifefish will use sonar to hunt for mines. The Knifefish could be joined by other 'bots, too — the Navy is purchasing a German-made underwater robotic vehicle to perform similar tasks — and while the dolphins and sea lions are out, divers will work alongside the robots on missions.
Don't bust out the tissues yet, though. According to the BBC, the Navy indicated that "there may still be some specialized missions where sea mammals are needed past 2017." Like what, you ask? Well, maybe this. (But probably not.)