Mark goes under cover as a carnival performer to investigate the drowning death of a top government agent; guest Billy Barty.
A pirate (Neville Brand) kidnaps a sea siren and forces her to use her hypnotic song to extract secrets from a defense expert.
Staffers of a Navy sea habitat meet a playful water-breathing imp (Pat Morita) whose touch causes them to revert to childhood.
Con man Jake Muldoon (Ted Neeley) finds a two-headed sea monster and sets it up as an island god as a way to collect tribute.
C.W. ingests an oceanic enzyme, causing him to split into two personalities -- one normal and the other careless and criminal.
Underwater turbulence transports Mark to 14th-century Italy, where he winds up between the feuding Montague and Capulet clans.
Mr. Schubert's search for radio-transmitting crystals reveals the existence of an underwater world protected by a force field.
Seeking the cause of a strange pain, Mark finds himself in the Old West, facing a look-alike (Patrick Duffy in a dual role).
Mr. Schubert threatens to attack international swimmers with a 10-foot-long mutated jellyfish if he is not paid $1 million.
A fast-talking swindler (Ted Neeley) leads Mark to the gold-mining operation of an otherworldly giant (Kareem Abdul-Jabbar).
In an underwater cavern decorated with hieroglyphics, Mark finds a hawk artifact with a vial of anti-energy banded to its leg.
Mark must find a way to communicate with a semi-intelligent machine Mr. Schubert has built to mine minerals from the sea floor.
Mr. Schubert threatens to melt the world's icebergs and cause widespread flooding unless Mark surrenders himself voluntarily.