Well you have two options....New Harbor (closer to the mud slides and full service fuel) on the West side of the island or Old Harbor (closer to center of town) on the East side of the island.
The very limited number of slips and moorings in Old Harbor make it difficult. The slips do not offer floating docks which is a pain. Depending upon the size of your vessel and the crowds you may be 3 deep (as in, two other vessels tied up along side of you). You can also opt to anchor behind the breakwater but you need to make sure you're deep enough to stay afloat at low tide but close enough to be out of the way of the large ferries. It has been two years since I have been to Block Island but there are (busy) showers/bathrooms available on the dock. You need to buy tokens from the dockmaster.
New Harbor offers more boating services and facilities. There is also an area where you can throw anchor and call a water taxi. There is a lot to do there but you will need to take a taxi or be up for a good hike to get to the center of town. New Harbor offers floating docks as I recall but verify that.
You're looking at about $100 a night for a slip for a 25' boat.
Fog and weather can roll in thick as you know in the Block Island Sound. Have more than 1 GPS aboard and all the other usual safety devices (charts, charting tools, comfort in using them, good fixed mount VHF with 8' antenna, etc.)
Have fun! I have spent Mem Day Weekends there when there were zero crowds. July 4th weekend is another matter - it's a zoo with the punks howling at the moon all night long.
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Captain Matt | 
