As mentioned in the previous comment, stay well within the markers in Poole Harbour, especially in the narrow channels and creeks. Many of these are dredged to the width of the markers, so veering even 1ft outside them could ding your prop or, worse still, leave you beached on a steep bank. This is still a risk irrespective of tide height in some places.
So, keep within the marked channels and have a great weekend, the harbour is a wonderful place to explore.
Happy when wet!