Sig Winguard (Monster), Hubie Kerns (Crewmen)
This is Wayne Heffley's second outing as the Ship's Doctor.
This is the second time the monster guest stars on this series, the first time being the first season episode "The Condemned" in black-and-white. It would also show up in several episodes of Lost in Space.
Dobbs and Hawkins are spotted through the periscope floating on the ocean. Shown an image of them on the monitor, Crane asks for a close-up--but the resulting camera angle is roughly 90 degrees from the original.
The same tall, thin, brown-haired man is seen as a common crewman (helping carry Dobbs into Sickbay) and later as a uniformed officer (standing by while Nelson douses a fire in the ballast controls).
(Nelson and Crane spot their runaway Flying Sub.)
Nelson: Dobbs and Hawkins.
Crane: They'll kill themselves.
Nelson: I'm not worried about them. I'm worried about the guidance device.
Although not listed in the credits, Allan (Riley) Hunt had a speaking role in this episode.