This has probably been posted before so sorry if Im repeating old news..
Basically, where the boat is (in the naval yard) that you steal the hydra from in vertical bird, swim to the front end of it, then swim toward the dock where it's moored to get 5 stars, then turn around, dive, and swim away.
Do this a few times.
Then have a little look around you.
Dead cop bodies, floating EVERYWHERE!
They spawn in midair near you so just fall into the water and drown
I panicked and took a crap photo of a few of them before they disappeared >_<