Sky 1/Flying Sub 1.

Several points worth pointing out, between Sky 1 & Flying Sub: The Flying
Sub was shown re-docking with Seaview on several occassions, and it was
also held in its own Silo (like Trident/Polaris missiles), and also the
Silo was permanently full of water.

These would probably make the Flying Sub more "believable" than Sky 1.