The episode "Operation Crash-Dive" was originally entitled "The Test Crew".
This episode is the first time Thunderbird 2 is seen dropping its cargo pod into the water to launch Thunderbird 4.
The hydrofoil, seen as a rescue boat here, and the equally futuristic hovercraft (seen in some James Bond films) were considered state of the art seagoing craft at the time. They were extensively used as cross channel ferries in the UK, but fell from favor in the ensuing decades.
One of the few times the off duty Tracy sons are seen smoking cigarettes (when playing billiards), though Virgil's 'Operation Cover Up' portrait appears to show him holding a cigarette, too.