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.
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. :)