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.
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.