The American Power Boat Association has many classes of racing for three-point hydroplanes and runabouts, with a large span of engine sizes in both stock and modified forms.  Their web site has a wealth of general and technical information about racing classes and motors.

Hydroracer.net is a racer's forum with information on upcoming races, equipment for sale, and  a source of helpful information for newcomers to the sport.

The SOR Racing Team competes in only four of the many classes, and some introductory information on the motors raced in these classes are provided below:

Stock Classes                                                Modified Classes                                          
B Stock Hydroplane (15 cubic inches)                      250cc Modified Hydroplane
20 Super Stock Hydroplane (20 cubic inches)       350cc Modified Hydroplane