The coastguard was alerted to a swimmer on Friday afternoon (July 15) who had been shouting for help off Pagham Harbour.
Both Selsey Lifeboats searched the area as well as the Coastguard Helicopter Rescue 175.
Coastguard Rescue Officers carried out a shore line search along with Littlehampton Coastguard Rescue Team.
The swimmer was located lying on the beach after managing to make his own way back to the shore.
He was still conscious but exhausted and had swallowed a lot of sea water.
He was then taken to St Richard's Hospital by helicopter.
Nick White, Littlehampton RNLI Lifeboat Station’s Lifeboat Operations Manager, said:
"The weather is absolutely glorious, but we want to remind everyone to stay safe at the coast. During this ongoing heatwave, the sea and the river look especially inviting, but it’s important to know the risks, whatever your experience or ability. The water is still cold, and the River Arun is one of the fastest flowing rivers in the UK.
"If you are planning on going to the beach, we would encourage you to visit a lifeguarded beach and to swim between the red and yellow flags. If you get into trouble in the water, Float to Live – lean back, use your arms and legs to stay afloat, control your breathing, then call for help or swim to safety.
"If you’re heading out on the water, check and double check that your vessel is safe, secure and seaworthy. In a coastal emergency, call 999 and ask for the Coastguard."
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