I used Redcliffe for some years with my previous ( fibreglass) boat..
I had 300mm draught, and on neaps it was manageable at lower states of tide, with care. Need to follow the buoys and stakes as you reach the River Frome at top end of the harbour, and slipway is a bit ropey with stones at bottom end on low tides. It is a good run down to Poole and Sandbanks, but some good open water.
Facilities are basic, with toilets on camp site, and plenty of room to park, and prepare boat.
The sailing academy at Portland is good- all facilities ( toilets and wash down facilities at the ramp) good parking, and cafe in main building. Not sure about overnight parking etc, but worth phoning them. Good, protected access into harbour. Can be Strong ebbtides in northern entrance to open sea. Great for Weymouth, the bay, Lulworth and Portland Bill