The trick is to argo one, have him chase you into an underwater cave where he will not be able to fit through the door but his long neck and head does. when you do this he will not de-agro, you can then get a clear shot from the cave with a bow and tranq arrows
These guys are great and super fast and also very strong. Warning when your gonna go tame these guys bring a sarco or an icthy , they also spawn In 2,s or 3,s so be careful because these guys do a crap ton of damage .
They can't fit through the resource caves' entrances. Just lure it into one and pull out your crossbow/harpoon gun for the knock out. Just be aware of any nearby megalodons, as they can fit into the cave easily and ruin your swimming session.