Try Ridge Farm Campsite, Wareham, for Poole Harbour, Studland etc. It’s well up te Wareham River (check Google map) so outboard, oars and/or lots of mega short tacking are the order of the day but the harbour is superb and you can do Studland and Swanage. Check tides before booking – there are weeks when there is a double high that lasts most of the day for several days.
We have stayed at Park Coppice caravan club site nr Coniston. Caravans and camper vans. This is well back from the shore but boat launching was possible a few years ago. Ring them. Between this and the shore is a tenting site with a long beach on the lake and quite a few boaters. We don’t know the name, but if you Googlemap Coniston and pan southwards from the lakeside jetty and cafe you will see it. Peel Island (Wildcat Island) awaits!
Check NW Scotland coastline for several tiny beach type sites, e.g. Clachtol Beach