If you are planning on doing anything involving water prepare to despise these flat horrors. They can out swim you. They can swarm you. They can act friendly and once you turn your back eat your tame. Beware.
Tame a kaprosuchos and swim to one of these bad boys and do it's alternate attack it will grab it in it's mouth. walk onto the shore where its safe and have another person feed it while in the gators mouth (if you unmount it drops it) it wont attack or die because it's technically not on the shore.
If caught in open water by a manta fight don't flee. If you have a spear or pike swim up as it gets close. This keeps you above it attack area and puts you in striking range. The manta is slower moving up and down in the water.
The manta is quite dangerous, as it's attacks can punch straight through armour, even flak. They are very fast and unless you have lots of oxygen and speed, you will not escape. Just poke your head underwater and look left and right before crossing the sea. It's like a road, but worse, because roads don't have angry stingrays, very large sharks and/or killer piranha
Mantas are very quick ocean transport mounts, which also double as a powerful PVP asset. The manta's attack ignore the armour stat, which makes a high melee damage manta incredibly powerful to players and small tames. It's quick speed also allows it to move very fast. It can jump out of the water, but on contact with land, it will begin dying very fast. Very good for travel on the center map, as it can jump over the many islands if it gets a good run up
So, yesterday I tamed my first Megalodon (around lvl 105), I went outside of our base to test it. Killed a bunch of other megalodons and mantas. That was pretty easy. Went back to base. An hour later, my megalodon died...ONE OF THE FUCKING MANTAS FOLLOWED ME AND ENTERED VERY SNEAKY IN MY BASE AND KILLED MY FUCKING MEGALODON. It was a lvl 5 so it took a fucking hour to kill my megalodon. Those are bitches. Update: I tamed another megalodon today. Lvl 200. Fuck mantas.