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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Wednesday lunchtime's weather for the east of the region
Wednesday lunchtime's weather for the west of the region
There is a high risk from birch pollen during any good weather. Oak pollen is also on the increase.