If you encounter a sarco TAME straight away these things have good swimming ability and great melee damage,it’s also quite cute once you get used to it (use narcotics and tranq arrows to tame) EATS MEAT!
In my experience I have found a couple of ways of taking these big boys down easily. The first one is to utilise it's turning circle as it cannot turn fast at all, so hug it's back legs, turn with it and whack it with some sort of melee weapon at the same time, a sword or pike works best for this. The second technique is to just walk backwards and shoot it with something, preferably something damaging, so I wouldn't suggest using a slingshot here. But doing this back step technique will allow you to just be too far away from the Sarco's bite, however, be careful of what can come sneaking up behind you. And by all means do not try and out swim this thing, it will win, every time.
When riding on a boat keep an eye out for these guys because unlike most other water dwelling predators, sarcos can still see you and attack you on the raft. If it gets to you it will be hard to shake off the raft, it'll make the raft move buggily, and if you don't act quick it will ruin anything you're carrying on the boat, and can of course kill you, most likely leaving you're raft stranded. If you have a partner have them stab it to death, they are weaker than they look. If you're alone, try to get as close to shore as you can and fight it off there.