tokobe has accurately described what to expect with overnighting in The Great Salt Pond. We have been cruising there for many years and will continue to do so with our next boat. Block Island is a wonderful destination and despite the party atmosphere generated by Champlins and the hundreds of powerboaters who summer at that marina, I consider it an ideal sailor's destination.
You're arriving on a Thursday or Friday at the end of June. My sense is you will have a difficult time finding a mooring on Friday, better odds on Thursday. We have never reserved a slip - can't even consider why we would do this. The anchorage conditions, with sufficient ground tackle and anchoring skills, is very good in most places, and if you cruise the perimeter of the public mooring field for a while, boats do leave on occasion.
I would suspect most slips will be filled on every summer weekends - arrive on short notice and Champlins, Block Island Boat Basin, and Paynes, will raft you alongside existing boats . . . at times five or more boats are rafted together at Champlins - the most greedy marina there. If you must have a slip, then BIBB is the best place to be - but they're usually booked well in advance. At this time of year, there may be cancellations, so you never know until you call upon arriving. Be sure to hang out at The Oar, the local sailor's watering hole/restaurant at BIBB.
Another favorite pastime is to dinghy over to Dinghy Beach - at the northeastern corner of the pond. Either beach or anchor your dink and walk across the sand bar/roadway to Crescent Beach - the best beach in Block Island.
Enjoy your stay - hard not to if weather is favorable. We'd join you there if our daughter wasn't getting married in Newport on the 28th.
True Blue . . .
sold the Nauticat