Hi there. We kept a boat in Langstone Harbour for many years and had the benefit of working at the VT yard in Porchester from 1978. It did not happen often but the sea surface at Porchester froze several times in that period. I would guess that Fareham lake would have similar problems as I would expect the sea temperature would get colder the further you are from the harbour entrance. While there is not a vast amount of fresh water entering the harbour, I would think the salinity in Fareham lake would be lower than the entrance and these 2 factors make it more likely to freeze. Might be a good idea to talk to people nearby who should know like Fairweather Marine
Friends who kept their boats afloat in the top of Chichester harbour overwinter did have problems with sea water circuits freezing and popping core
plugs etc. They used antifreeze to protect any fresh water circuits, drained down the calorifier
and fresh water tanks
and emptied the saltwater circuit as far as possible.
For a saildrive, all I can think of doing is attaching a dinghy
to the saltwater pick up line and blowing down to expel water and while blowing shut off the inlet.
The biggest problem will be the above water parts
of the circuit as the air temp will get far lower than the sea temp, but there is a risk that water in the saildrive could freeze in a very cold snap. Might be worth investing in GSM boat alarm
that can monitor
temperature and alarm
texts to your phone
. They are are about £150 on ebay