Fire crews, police, water rescue teams and paramedics were all called to the incident between Port Solent and Portchester Castle between 9.42pm and 10.22pm.
They arrived to discover a ‘despondent’ man in his 20s, from Paulsgrove, in the water without any clothes on – walking away from the shore.
A spokesperson for Cosham Fire Station said the man had wandered ‘between 100 and 150m out’, which at one point caused services to ‘lose sight of him’.
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In a joint rescue, teams spread across the shore and found him using thermal imaging cameras and torches.
Four Coastguard and Gosport Lifeboat swimmers then coaxed him into walking back to safety in a ‘self-rescue’, where he was met by police and treated by ambulance staff.
A spokeswoman for Hampshire Constabulary confirmed the man was then taken to hospital.