It was massive, over a metre across, gliding gracefully along the sand on the sea bed.
I found this information on them:
Mantas are extremely curious, are fond of swimming with scuba divers and pose no threat to humans. Although they may approach humans, if touched, their mucus membrane is removed, causing lesions and infections on their skin. They will often surface to investigate boats (without engines running). They have the largest brain-to-body ratio of the sharks and rays. Mantas are known to breach the water into the air.