Looking at the almanac (I haven't been to Christchurch by boat) it doesn't look great. High tide on Sat is about 11.20am at the entrance and around 11.50 at the quay (about midway between springs & neaps). The bar/channel is liable to shift(!) and the channel inside the harbour is narrow and mostly shallow (approx 0.3m) soft mud. Mean ranges are 1.2 sp and 0.7 nps. Floods and droughts cause considerable variation from predicted heights.
The River Stour is navigable at HW up to Tuckton and the River Avon up to the first bridge. Apparently there is a limited number of alongside berths for monohulls with a max LOA of 9m. Contact Christchurch Sailing Club on 01202 483150.
I hope someone else posts to either correct me or (miraculously) tell me I'm right!!!