They are often referred by the trademarked brand names Jet Ski, WaveRunner, or Sea-Doo. Most are designed for two or three people, though four-passenger models exist.
The United States Coast Guard defines a personal watercraft, amongst other criteria, as a jet drive boat less than 13' in length, in order to exclude from that definition more conventional sized jet boats. There is a wide variety of "jet boats" many of which exceed 30-40' in length.
Source: Wikipedia, YouTube