The megalodon is one of the easiest tames in ARK.

Simply use a crossbow with tranq arrows, and swim at it while strafing left or right. The megalodon, like most large underwater creatures, has a horrible turn radius, which is very easy to get inside of. Just tranq it till it runs, than tranq it till it stops. If your having trouble with this tactic, boost your speed or wear scuba flippers.