The problem with Northney, and the Watersport centre to a lesser degree, is the amount of time it takes to get to open water without a speed limit.
The Camber, as suggested by Dry Run, is ideal really as you've only to cross the harbour(Remembering to call the HM to ask permission) then you're out through the channel into the Solent in 5 mins.
The ECA is a good place, and if they'll let you join it has all sorts of other advantages.
I wasn't aware the public slip had been un-adopted by the council.
RIBBED For extra pleasure.
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