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.
This thing will attack you through armor. I got my 500 armor and 700 durability FLAK CHESTPLATE rekt by this little son of a.... Basically armor doesn't work against this creature, might as well kill it.
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