I'm a newbie to this site, after just taking delivery of a new RIB. I have though, been the proud owner of 4 SIB's from Zodiac, Honda, Suzumar and Lodestar, and wanted to back the suggestion for Redcliffe Farm.
I stayed there on a couple of occasions during the Summer of 2011, and found it a great place for sibbing. It is at the top of Wareham Creek and allows a great mix of gentle river cruising, exploring the harbour and venturing out across the bar to Shell Bay and beyond to Old Harry Rocks. Swanage is also within easy reach on calmer days.
The best bit is that even if the weather is nasty, Poole Harbour doesn't get much beyond a chop, so anything bigger than a 3.2m would handle it with ease. There is a 10kt speed limit but again, most SIB's can plane easily at that speed.
There are some great channels to explore in the Harbour, and scenery is great, including seals, yes, I said seals, around the Arne Channel. The campsite itself is a gentle 10 minute stroll along the river from Wareham town, with excellent pubs etc.