Almost anything but a Baryonyx is prone to die if left alone with a swarm of Mantas, including Plesiosaurs, Basilosaurs and Tusoteuthis. In the wild, those creatures are the third fastest you can encounter and will ignore armor when attacking. Absolutely letal if combined with Cnidaria.
They are notably common on The Center, to the point where you can clear a shore and only let the one you want to tame but still get up to three following you to your trap once you decide to lure it.
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.
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