They were paged to say that a car was in the water at Broadmarsh Slipway, which is at the north end of the harbour, off Harts Farm Way in Havant.
Police were also called out, said the coastguard. It happened at 1am on Saturday.
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After checking there was nobody in the car, coastguard officers waded out and fixed two lines to it to make sure it didn’t float off into the harbour when the tide came in.
The coastguard then remained stayed while the car was recovered.