A man has drowned and heroic beachgoers have rescued his son-in-law from the ocean after the pair was thrown from their boat by a wave.

The two men were thrown overboard at about 7pm yesterday by a large wave at Christies Beach, south of Adelaide, just 10 minutes after launching from the O’Sullivan Beach boat ramp.

A family on holidays from Victoria first noticed the trouble when they saw the duo’s empty vessel had washed ashore.

“I could hear somebody out there needing help and the boat was the quickest way to get out there,” Richard Tickner said.

He and another local jumped aboard the sinking vessel the men were thrown from, which had washed up on the beach, and rushed to save the man.

“It was rough, it was choppy, there was quite a bit of water in the boat so it wasn’t floating too good,” Mr Tickner said.

“It’s pretty difficult getting someone who’s got no energy left over the side, (but) yep, we got him in.”

The 41-year-old Golden Grove man was taken to the Flinders Medical Centre, and was released this morning.

His 65-year-old father-in-law, of Hallet Cove, drowned in the ocean. His body was found at about 8.15pm.

Emergency crews on the beach following the drowning of the man and rescue of his son-in-law. Photo: Andrew Ward.

7News understands the boat had been bought only a week ago, but witnesses say it was too old and too small to handle last night’s choppy conditions.

Meanwhile, another man is lucky to be alive after his boat ran into rocks in the Port River channel this afternoon.

He was rescued by a group of three men in another boat that saw the vessel crash into the rocks.

Authorities say the Christies Beach tragedy should serve as a warning to others.