Ross Elliot (Chairman), Al Shelly (Assistant) OR Alan Baxter (Assistant)
There's some confusion as to the identity of the Chairman's assistant. According to Science Fiction Television Series, it's Shelly, but in Irwin Allen Productions, it's Baxter.
After the drone sub attack, Crane goes to the radio shack. He distinctly tells Sparks that he has a message to send in code. Sparks gets out his pad of paper and begins scribbling--presumably taking Crane's dictation to afterwards translate into code and transmit. In the following scene, the enemy is listening to a straight transmission of Crane's voice.
At the end of the episode, the killer confesses that he sent a dummy out in the mini-sub--but in the scene with the mini-sub, you can see the "dummy" blink.
In the actual credits of the episode, the writers listed are Michael Lynn and George Reed, but these are pseudonyms for the writing team of William Read Woodfield and Allan Balter.
According to the book Irwin Allen Productions, the writers William Woodfield and Allan Balter had a falling out with Irwin Allen after Allen wanted the killer in this episode to be an invisible alien. Woodfield and Balter left the show and they ended up filming the original ending anyway.
Some of the "battle stations" footage comes from the movie "The Enemy Below".
The submarine attack in the episode's opening scene reused the sub attack footage from the 1961 movie.
And Then There Were None (1939)
The plot in this episode bears a resemblance to the novel "And Then There Were None" by mystery writer Agatha Christie. In the story, 10 guests are invited to a a secluded island by a mysterious host and summarily killed one by one. Here we have the stalled Seaview serving as the island and the 10 technicians serving as the guests, with Nelson and Crane as surrogate hosts/investigators.