Eleven-time world champion Kelly Slater has seen a lot in his surfing career.

But nothing compared to the incredible scenes that played out during the Margaret River Pro today.

Slater was among the many big names who sat back to watch the round three heats when a pod of at least five dolphins joined American surfer Conner Coffin's barrelling wave.

"It was insane," Slater said. "There were like six dolphins in the lip. I thought one was going to get pitched.

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"That was one of the coolest things I've ever seen in my life."

The incredible ride with dolphins helped Coffin outscore Jesse Mendes 12.33-9.20.

"I just got that wave, and then I looked up and there's a pod of dolphins right by me and a perfect blue set coming," Coffin said.

"I thought, 'Oh my god this is insane'. I was really excited."

Meanwhile, two-time world champion and former Margaret River champion John Florence crashed out on his first wave of the day.

Recovering with a series of solid barrels, however, the Hawaiian surfer posted a two-wave score of 11.83, edging past Australian Jack Freestone (9.16).

"I wouldn't say it was one of my best heats, but it's always nice to make it through the close ones," Florence said.

"I was just bummed I didn't get barrelled more."

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Slater also pulled off some smooth rides to beat Willian Cardoso 10.06-7.34.