Up to 25 firefighters are tackling the blaze at the yacht club in Temple Manor Marsh, Knight Road.
It started at around 8pm.
A fire launch boat and the Coastguard are also at the scene.
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Thursday Evening's Pollen Forecast
Southern Health NHS Foundation Trust has appeared in court to enter a plea to the charge that it failed to provide safe care and treatment.
Thursday afternoon's weather for the east of the region