There is no historical record of a roller coaster disaster at Rocky Point Park (Warwick, RI) sending cars into the ocean.
Rocky Point (operated 1847–1995) had amusements, but its major rides were a carousel, Ferris wheel, and later a log flume—never a roller coaster. The “cars in the ocean” story appears to be an urban legend or confusion with another park (e.g., Seaside Heights, NJ, or fictional accounts).
If you’re researching a specific incident, verify with the Warwick Historical Society or Rhode Island state archives.