The Flying Sub was introduced in the 2nd season. I always felt that this was one of the coolest miniatures of the Irwin Allen series, followed by the Jupiter 2 space pod.
There's quite a cast of guest stars in the first 15 episodes: Eddie Albert, Werner Klemperer, Lloyd Bochner, Jill Ireland, Steve Ihnat, Nick Adams, Yvonne Craig, Everett Sloane, Michael Ansara, James Doohan, Malachi Throne, June Lockhart and Caroll O' Conner. We also get the aforementioned great episode "Submarine Sunk Here" and Harlan Ellison's "The Price of Doom" (who appears in the credits as "Cordwainer Bird"--the pen name he used when he was disatisfied with changes made to his scripts, an inside joke of 'fipping the bird'). Some nice scores here also by the great Hugo Friedhofer.