Great Salt Pond has a huge anchoring area North of the channel if that is the way you want to go. We did a night sail from Newport a few weekends ago, picked up a town mooring, and then started calling marinas at7am and got a slip at the Boat Basin. (My favorite as they do not raft boats.) Payne''s usually has space if you call them in the am as they do not take reservations. You might also luck out and find an empty town rental mooring (lime green in color). Good luck.
Great Salt Pond is one of the worst anchorages anywhere. The east side has a very hard bottom and burying an anchor is almost impossible. Boats drag there all of the time. It costs $500 there now to have your boat retrieved.
I have a private mooring there so I am somewhat secure except from the anchored boats coming down on us. We were just ready to leave for shore when a motor boat tangled his prop on the anchored sailboat that was just to windward of us. I just called the fun police and left. I have seen it all.
Our friends were anchored on the east side and a boat dragged on top of them. My friend is very big and strong and he did hurt his himself however keeping that boat off till help arrived.
Head straight for the town moorings. Beg someone who is on one to let you raft with them if you can''t find an open one. It''s late in the season however and you may find one. Labor day weekend will be a different story.
I don''t think I would leave a boat anchored there on just one anchor.