The Jersey Shore is an endless stretch of sand facing east stretching 100+ miles. RI's coastline is mostly rocky and small cliffs, with the beaches in between facing south or east. The towns I mentioned, with the exception of Narragansett, are more beach than rock. Narragansett and Newport is more rock than beach.
In Rhode Island, people who go to the beach are there to relax on the sand and swim. In the afternoon, they go home, not to the bar or nightclub. When I went to the Jersey Shore with college friends, most of my friends had no interest in the actual beach or swimming, they were there for the bars and nightlife. I couldn't drag them to the beach. Makes sense because many of them can't swim.
Rhode Island is more about the sand and water, the Jersey Shore is more about the entertainment.
Like NJ, RI beaches and sea floors next to the beaches are sandy.
Not sure what beach you went to in Massachusetts. If it was along the Cape Cod National Seashore, the water temps there are 5-10 degrees colder than the Rhode Island ocean beaches. Right now, Cape Cod National Seashore is in mid 60s. Rhode Island is in mid 70s, which felt like a record high when I was there this weekend.
The NCEI Coastal Water Temperature Guide (CWTG) provides recent ocean and Great Lakes temperatures and average water temperatures collected from buoys and other monitoring stations in the United States and its territories.
coastalwatertemperatureguide-noaa.hub.arcgis.com