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
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.