I've spent short summers at Nantucket - (aside from never finding someone with a boat) did not find much in the way of a shore dive. There's a lot of sand out there. I did manage to get in a fair amount of snorkeling among the eel grass beds. Scallops, pipefish, etc abound. This can be endless fun.
There's was a tourist shop in town (probably still is) which had a compressor. The owner is a complete _______ and is quite happy to scream at people. Employees, as well as customers. Some off eight years ago air fills were going for something $15.00 for an 80 cu. I can't imagine what it is now. That said they were the only game in town.
There are other things which are a lot more fun - and uniquely Nantucket. Cycling out to the different beaches was one. The other was going to the Turtle pond and seeing the giant snapping turtles and blue crabs. A third was the garbage dump where treasure of all kinds were being recycled. There is also teeny little aquarium in town run by the Maria Mitchell museum.
Best,
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