Potter cove is located just North of the Newport bridge, almost under it, with a Northerly exposure. It's one of those sites that gives you an opportunity to get into the water when the nicer South-exposed sites are blown out. On days like that Potter Cove tends to be pretty smooth and glassy, even in strong Southerly winds.
If you're diving Potter Cove, be prepared for a rather long surface walk, rather than swim, especially at low tide. It does not get deep enough to submerge until about 100 yds. out. Beyond that you'll usually hit about 15-20ft. The bottom is sandy, but the sand is coarse and does not get stirred up as easily as say Ft. Wetherill, so visibility can be pretty good. Despite the bottom being mostly sand, there's a variety of sea life - lots of spider and hermit crabs, starfish, and various other fish that you'll see along the bottom. It's also a good place to dive for bottles, there're plenty of them down there, some so encrusted that they're barely discernible. Entry is pretty easy, right off the stony beach, and there's plenty of parking. Overall, it's not as bad a site as some have made it out to be, and you can make it a rather enjoyable dive.
Ft. Wetherill is probably the most popular dive site in Jamestown. There are two coves to pick from, the left and the right. They both face South and are pretty well protected from most but southerly winds. Best diving is along the walls in the coves - there is plenty of variety in terrain and sea life to keep you coming back for more. The bottom drops off fairly quickly, so you can get some depth out by the points. It is rocky along the walls, but mostly sand in the middle of the coves. Another draw to the site is that it is known to harbor tropical fish that occasionally make their way up into the New England waters.
If you come to dive at Wetherill on a weekend, try to come either really early or later in the day. Due to its sandy bottom and ease of entry this site is often used by OW classes that stir up lots of silt and sand, thus greatly reducing the vis. Also, if the winds are blowing out of the south, expect rougher seas in the coves and visibility that is very likely to be near nil. On a nice day, however, it can be a very reasonable 10-15, if not better.
Entry to the sites is easy. If you park in the main lot between two coves, the right cove has a boat ramp that you can use, and the left a small beach. You can also drive a little past the entry to the main park, and park over by the Coast Guard facility near the remains of the fort. There is a tiny secluded rocky beach at the south-west point, below the remains of the fort, that you can use for entry.
Equipment: If you got an underwater camera - bring it. You never know what you'll see, and as I said above, tropical fish are often seen taking shelter in the coves.