Probably One of THE Most Impossible Questions To Answer [I'm not holding my breath]:
When I was a kid I watched a movie on television (not sure if it was on Cable or Broadcast TV ) about this group of people who were stranded at the bottom of the ocean in a sunken vessel for many years. So long in fact that they created a community there.
While down there for all those many years the people developed a way to survive and even prosper.
I'm not sure of the specifics but someone comes into the closed community, I believe a man or a group of men. The people are offered the option of rescue but they don't want to go because they are happy to have a connection with nature but a girl fell for an outsider and wanted to leave with him (I think... it could have been the girl was part of the expedition and was looking for her dad's ship, I'm a little foggy on that ).
The girl's father was a leader in the underwater community and tries to stop her from leaving. Really that's the most memorable point I recall... that and perhaps an underwater garden of some sort on the ship.
It's all the typical tropes but for some reason this story stuck with me and I'd be interested in seeing it again so if anyone can direct me to this it would be great. I don't think it was from one of those "pseudo Sci-Fi shows" like Sea Hunt, but I could be wrong so if you recall something like that don't count it out. I am sure it was in color though.
All I can say aside from the above is that it must have been made between the early 70's and late 80's... I watched a lot of TV as a kid so it could have been on almost any station from HBO to those local stations that are now part of the CW and Fox networks who would do Sunday Afternoon Sci-Fi/Monster Matinees.