Bethany Hamilton's three-year-old nephew found unconscious in bath

Bethany Hamilton has pleaded for a medical miracle after her three-year-old nephew was found unconscious in a bath.

The pro surfer took to Instagram to explain the accident occurred on Friday night and the toddler was airlifted to a hospital in Oahu, Hawaii, via helicopter.

"He still has a heartbeat and has fight in him," she wrote alongside a photo of her nephew holding a surfboard.

"We are wrecked," the shark attack survivor added. "But I know how proper medical support can make or break someone's chance of survival.

"In this case we're asking for help from anyone who has information of what we can do to give my nephew the best chance," Bethany continued.

Bethany's brother, Noah Hamilton, referred to his nephew as a "resilient kid" and an "amazing swimmer", as reported by the Daily Mail.

"He's three-and-a-half going on five," Noah said. "He's a strong resilient kid and an amazing swimmer. He was found not breathing in the shallow water of a bathtub. His vitals were enough to get him to Oahu."

Bethany later gave an update to her followers, explaining she had been in touch with Dr Paul Harch, a hyperbaric oxygen therapy clinician.

Bethany inspired the 2011 movie Soul Surfer starring AnnaSophia Robb. The movie told the athlete's story about how she lost her arm after being attacked by a tiger shark off the coast of Kauai in 2003 when she was 13 years old.