Mystery surrounds the death of a man whose body was found floating in the water at a beach south of Adelaide.

Two young boys about to go fishing made the discovery near the O’Sullivan Beach boat ramp just before 10.30am on Thursday.

WATCH THE VIDEO ABOVE: Man’s body found floating in water near boat ramp.

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The 29-year-old man from Christie Downs was taken from the water and brought to shore but could not be saved.

Joshua, one of the boys who made the discovery, told 7NEWS he initially thought the man’s body was a pipe floating in the water.

“Me and my mate were just about to go fishing this morning and then we found something that looked like a pipe,” he said.

“Then we had a closer look at it and we found a body with a skateboard attached to it with fishing line.”

The man’s death is not being treated as suspicious but the circumstances remain a mystery.

Police are preparing a report for the coroner.