I just wanted to say thanks for this thread! I had a vague memory of seeing an ad for a movie on TV when I was a kid that was about people trapped underwater in a boat for many years, but I couldn’t remember anything about it. I eventually convinced myself that what I had seen was an ad for a TV broadcast of The Poseidon Adventure, and I had misunderstood the plot. Because, like the OP, the plot I thought I remembered was so bonkers that I decided I made it up.
A similar movie was Airport 77, where a 747 crashed into the ocean in the Bermuda triangle (of course, it was the 70s), then sank to 100 ft, but was somehow water tight, so the passengers mostly survived. They were down there for hours, not decades, but still utterly implausible. It does have a ton of great actors though - James Stewart, Christopher Lee, Olivia de Havilland, Jack Lemmon.
Three times, I think. IIRC, an character says “It’s lucky we learned to launch these quickly.” Totally absurd.
That’s the one! I remember the bit with the coffin and the part with the flaming passenger-way.
In fairness in the '70s everyone tended to wildly over estimate the space shuttle’s capabilities and how it would revolutionize access to space.
It’s a sign that the makers knew that the Shuttle doesn’t launch that quick, but hung a lampshade over it.