So many questions, JBG! To answer in general terms, there are lots of places in New England where you can make your way through wild forest for miles--as Thoreau said, "no man's garden, but the unhandselled globe". (Hmm, spell checker objects to a word there.) Sometimes it could be National Forest, or state forests, or there's a fair amount of territory owned by nonprofit foundations too, such as the Harris Center. Bodies of water are harder to find, and most of them are pretty badly handselled. As you can see on the topo map, Nubanusit Lake has houses on it, though not a huge number and all along one side. I'm afraid the norm is for any body of water capable of floating a duck to be a magnet for development, and the places that have escaped tend to be in state parks, with trails around them and often ramps from which power boats ravage the silence. I agree that there's some exploring to be done around Spoonwood Pond, but if one has already driven 2 hours up from Boston, and then paddled a boat for an hour, with the prospect of the same to achieve in reverse, one doesn't tend to do long shore-based side trips. If we're willing to travel farther, like to central Maine or the Adirondacks, there are chains of pristine lakes with carry trails between them, and campsites waiting to be enjoyed.
Yes, the water gets to be very comfortable for swimming by midsummer. In most locations, I don't find mosquitoes to be a problem--the worst I've experienced was in Colorado! My avatar picture was taken at Ledge Pond in the Adirondacks, which as I recall we got lost reaching and then again on the way back. Here's another one from Junior Lake, Maine, on a trip my wife and I did. Notice that the boat's full of all our camping stuff.
If your travels could ever bring you this far, of course we'd be happy to see you, and we'd make an effort to show you a good naturist time!