The Beach Girls and the Monster (1965)

A young girl is killed at the beach in Malibu. Professor Otto Lindsay suspects that it is some form of mutated fish. However, his son Richard, who was a good friend of the girl, thinks that it is a madman who has a grudge against Richard and his friends. Soon the list of victims grows to include one of Rich's surfing buddies, Rich's bitchy stepmother Vicky and his friend Mark who was crippled in an auto accident.