Carlin's name is spelled "Karlin" throughout the story. The "K" spelling was used in pre-production material for the TV series, but was changed to "Carlin" when the series began production.

This story introduces a new ship to the SHADO fleet, the unmanned Sea Scope, a sort of seaside version of SID, able to increase Skydiver's communications reliability, guide it to weather-safe spots from which to launch Sky 1, and pinpoint termination points for UFOs over the sea. Unfortunately, it is destroyed here as well. Since the ship was unarmed, Straker seems to come to the conclusion that another would require full-time guarding to protect it from UFO attacks and shelves the idea. But why not arm any replacement vessel so it can protect itself? Based on this story, it seems it would prove itself nearly indispensible over time.

Page 56 reveals that when Sky 1 is separated from the rest of Skydiver, the submersible portion is referred to as (you guessed it) Diver 1.

As a UFO attack on Sea Scope begins, Carlin reflects on how the aliens uncannily always seem to know of SHADO's latest secrets. The aliens' intel regarding SHADO is a recurring theme throughout the series. It's never really explained how they are able to recruit or brainwash SHADO personnel so easily for spying, sabotage, and other clandestine operations, adding to the aliens' mystery.

Why does Foster race out to pilot the Shadair Rescue Freighter? Since the plane had recently been prepared for takeoff, then put on hold, shouldn't the original pilot still be closer (and, to be frank, more expendable) than Colonel Foster? 

UNANSWERED QUESTIONS

Sea Scope is said to be an unmanned, seagoing version of SID and have a computerized voice. Does the voice sound the same as SID? SID's voice was provided by Mel Oxley, the voice of public service messages for BBC radio and television in the 1960s.