[casstanet]

From 40 years ago, or more, maybe? Small group of people figure out how to survive for years (more than one generation) in a sunken vessel. I remember they used a bicycle for generating electricity. Seems like "below" or "under" was in the title, but I can't find by searching those terms. Not the Blish story.
Thanks for offering your time as an expert!

[Wondergirl]

A Young Adult fiction?

Call your local library and ask for the reference department. They have reader's advisory books (and memories) that will help answer your question.

[Lucas2009]

Not sure about a book, but there was a movie with that same plot. It was called Goliath Awaits.

Goliath Awaits (1981) (TV)

[sleemet]

I just watched Goliath awaits, and when looking it up I ran into this question and possibly also into the answer to it. Were you referring to:

James White:
The Watch Below (Ballantine U2285, February 1966).
A survival-kit novel that 'anticipated' the awful made-for-TV movie, Goliath Awaits, by fifteen years. In the first plotline, a Second World War British tanker is torpedoed and sunk -- but not all the way down. Some few people survive and even thrive in a huge air-pocket, knocking together their own life-support system. Much later, aquatic aliens from a dead planet splash down in the Atlantic and help free their unfortunate counterparts.
Other collectible editions: Whiting & Wheaton hardcover, London, 1966; Corgi GS7759, 1967; Walker hardcover, New York, 1969.

Found this on James White Information