Don't be dumb just get a boat and shoot it from there. If it catches you stab it with a spear or pike it's your only hope. Also don't try to out swim, unless you are level 100 and you have a sick water creature. :)
Sooo I tamed a sarco and named him Scar Scar. And then I accidentally set it to aggressive (finger slipped off the button while aggressive was selected). Then, you know, obviously he went on a killing spree on land, and then he went into the water and killed two manta. Then he decided to set his eyes on one of these, and well it didnt turn out good. Poor Scar Scar. Lost but never forgotten, you fought well little buddy.
I had a megalodon called jaws once, and I put him on wander to mate with another megalodon I had, but I forgot I left him on aggressive and he went after a dodo. He got beached and eventually about 4 dodos ganged up on him and pecked him to death. By the time I was there it was already too late. RIP JAWS ~ death by dodo