The US Navy uses sea lions and dolphins to patrol US interests here and abroad. You can read about it here...

There's a concern that California sea lions might not be able to handle colder environments such as Alaska. What I don't understand is why the Navy hasn't thought about using harbor seals. Harbor seals are just as smart as sea lions, and just as trainable. They can dive to deeper depths and can withstand much colder temperatures.
Sure, harbor seals may look like fat lazy buggers who sleep all day (and some of them can pull that off without any help at all), but they're every bit as much fun as sea lions. Harbor seals are awesome. They can drink any human under the table and they throw the coolest parties. One time this harbor seal invited me over to his house and we were trying to take his new Mercedes off these crazy jumps. I accidentally crashed it into a wall and the harbor seal totally didn't get mad at me. Instead, we went to taco bell and spent the rest of the day watching Benny Hill reruns on his widescreen TV.