You're right, realistic maybe wasn't the best word. I think "believable" would have gotten my point across better. It's true that GTA V mimics the real world pretty terribly, but it's game world and AI is so incredibly internally consistent that it feels less like a game and more like a another world.
> I accidentally bumped a police boat with my jetski. Apparently this warranted being shot in the head.
Were you playing as Franklin? The GTA V devs have denied this, but from personal experience and plenty of online reports, the police in the game are racist. More realistically, it's probably because they're unable to pathfind to a state where they can arrest you (because you're both on boats on water), so they just go guns blazing.