I came across a guy stood near the water, by the part where you end the last mission, and I decided to punch him and push him in. Now, I'd done this before and the guy had just swum and climbed up the ladder, so I thought I'd do it again. But I hit this guy a couple of times and knocked him in the water, jumped in after him and.. he drowned. He did the whole "arm in the air" thing that Claude and Tommy used to do. Pretty cool how certain NPC's can swim and others can't.
I was doing the police missions as well and I killed two guys in one car, pulled out the driver and then drove around with a dead body in the passenger seat.