You are really going to struggle with that request in the Swanage Area. The main slip in Swanage is ok but not a campsite for several miles. Are you looking for a site with a slip in the site?
Near Swanage you could try the Studland boat yard, it's got an easiler slip but it's no use at low tide. Still no camp site either.
The nearest I can think of is Rockley Park, that's at the opposite end of Poole Harbour. Good Slip, if you watch the cross current, and a huge campsite. But it'll take 40 minutes to get to Swanage by boat, speed restrictions in the harbour.
Hope someone can think of something I can't of or offer alternatives on the Solent.
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former --- Albert Einstein