Not sure if I have it correct, but if you look at the attached drawing. One on the left is the harbour as it is now, and the one on the right is after it'd been dredged. The blue bit is the new depth created by dredging. The red line is low water mark. The green line is high water mark.
The dredged area has no affect on the tidal flow thru the harbour entrance, as it is 'dead' water, it always sits there regardless of high or low water. The only water moving in and out is the top layer, from high to low water, this amount of water is always the same regardless.
There might be an effect if they dredge poole entrance as it will dictate the amount of water flowing thru, i.e., from x million gallons per minute to x+y millions gallons per minute.