The man had gone into the water off Brooklands, in East Worthing, yesterday afternoon to recover what he believed to be a canoe.
Despite the shallow water, his wife was concerned for his safety and called the coastguard.
The inshore lifeboat was launched at 3.40pm and it reached the scene in about 15 minutes.
The crew found the canoe was in fact a sailing racing marker snagged on the groyne. The man had come out of the water and was ashore, so the lifeboat was stood down and returned to station.