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.
For a low level you can use an ichy to lead one of these suckers to the shore and then just tranq it from land, just be mindful about any other water tames (or wild creatures) that might interfere. They're pretty slow and their bite isn't too devastating, but when you're swarmed they can prove to be deadly. I would recommend you knock it out in shallow water so you can keep an eye on its torpor without losing too much oxygen, and bring plenty of meat/narcs if you don't have kibble. They make good defenders if you have your tames in an open body of water and as long as you have enough of em
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. :)