A commercial lobster diver who got caught in the mouth of a humpback whale off the coast of Cape Cod on Friday morning said he thought he was going to die.

Michael Packard, 56, of Wellfleet, told WBZ-TV after he was released from Cape Cod Hospital that he was about 14m deep in the waters off Provincetown when "all of a sudden I felt this huge bump, and everything went dark."

He thought he had been attacked by a shark, common in area waters, but then realised he could not feel any teeth and he wasn't in any pain.

Michael Packard estimates he was in the humpback whale's mouth for about 30 seconds. (Photo / 123RF)

"Then I realised, oh my God, I'm in a whale's mouth ... and he's trying to swallow me," he said. "And I thought to myself OK, this is it - I'm finally - I'm gonna die.

"I was completely inside (the whale); it was completely black.

His thoughts went to his wife and children.

"I thought to myself, 'there's no way I'm getting out of here. I'm done, I'm dead'. All I could think of was my boys, they're 12 and 15 years old."

He estimates he was in the whale's mouth for about 30 seconds, but continued to breathe because he still had his breathing apparatus in.

Then the whale surfaced, shook its head, and spat him out. He was rescued by his crewmate in the surface boat.

"Then all of a sudden he went up to the surface and just erupted and started shaking his head. I just got thrown in the air and landed in the water," Packard recalled. "I was free and I just floated there. I couldn't believe ... I'm here to tell it.

"My first thought was I can't believe I got out of that situation. My second thought was for how injured I was."

His sister, Cynthia Packard, originally told the Cape Cod Times that her brother broke a leg, but he said later that his legs are just bruised.

Charles "Stormy" Mayo, a senior scientist and whale expert at the Centre for Coastal Studies in Provincetown, told the newspaper that such human-whale encounters are rare.

Humpbacks are not aggressive and Mayo thinks it was an accidental encounter while the whale was feeding on fish, likely sand lance.

- additional reporting Cape Cod Times