Use a high level Basilosaurus with high health to lead it to shallow water where the basilo gains health faster. Dismount and let the megalodon bite at your basilo while you traq it. Tamed 4 150 megalodon with this method works like a charm.
You are just wanting to go for a swim when suddenly you see a megalodon. You try to swim away but you can't out swim a shark. He will take a bite out of you and break a bone then leave but come back to finish you off with a final blow.