Rhys said they saw the dolphin jump on the woman’s back.
“We thought he was doing what he normally does – just a bit of fun,” he said.
But after hearing her scream, they paddled over and saw the dolphin repeatedly push her head underwater.
After bringing her back to shore, they returned to the water and saw the same dolphin targeting another woman, who was holding onto a yellow buoy while “looking panicked”.
Rhys said swimmers should try to get back to shore as quickly as possible if they spot the animal.
“I think it’s only a matter of time before the dolphin gets hurt or someone gets seriously hurt,” he said. “It is a wild animal, as exciting as it is.
“I think it’s up to everyone now to start respecting its space a little bit more.
“If we weren’t there that day I think something bad could have potentially happened.”