Watch the Baiter slip at very low tide it has a lip that catches a few out. You'll be on springs this weekend, the currents can be fast on lows in & around the Brownsea channels. I didnt think you're allowed to land on Brownsea as its a conservation area.
Keep to the channels in & out of the harbour, its is really shallow in a lot of places especially if you don't know it. I can vouch for the mud, spent many times stuck in it windsurfing
I would avoid the spring lows if I were you especially if touring the harbour.
Might be worth a trip to the pub at Wareham, a trip up the river is quite nice, its a couple of miles upstream.