Mr Smith, who has been going for swims in the harbour for more than 30 years, said he had noticed the temperatures getting warmer recently.
"It never used to feel warm," he said.
"You never used to be able to go in without a wetsuit but now you can."
The harbour felt a bit safer to swim in than at St Clair Beach because it was warmer and there was less marine life, he said.
Port to Port Cruises and Wildlife Tours owner Rachel McGregor operated a ferry service from Portobello to Back Beach in Port Chalmers and regularly kept an eye on the water temperature.
She said two summers ago, the water temperature reached 22°C. It usually hovered about 18°C, she said.
Ms McGregor said the Otago Peninsula side was warmer than the Port Chalmers side, which was warmer than Taiaroa Head.
