Portsmouth Lifeboat Station deployed its smaller vessel from its base in Ferry Road, Eastney.
Colin Hill, Hayling Ferry skipper, said he saw it launch at around 2.30pm after crew members appeared to search the mud nearby. Police arrived at about 3pm but have since left.
Mr Hill said: ‘It’s very strange. The lifeboat was launched about an hour ago up the harbour.
‘A police car turned up about 20 minutes ago.
‘Then we saw another police car with blue lights on the Eastern Road.’
The ambulance service said it was called to a medical emergency in Northney Road, Hayling Island, at 2.26pm.
Mr Hill added: ‘It looked like they were looking in the mud area.’
A search and rescue vehicle was also seen coming into Portsmouth.
The lifeboat went up towards Kendalls Wharf off Eastern Road.
A Solent Coastguard spokesman said units had been called out after reports of a person in distress in the mud of the harbour, but ‘an investigation reveiled (they were) not in distress’.
The individual did require assessment by paramedics at the scene.
A South Central Ambulance Service spokeswoman said: ‘(It was) a medical emergency that we were called to at 2.26pm.
‘We had an ambulance crew on scene assessing and treating the patient but no one was taken to hospital.’