I don't know why this is driving me so crazy. I only saw the ending, and the most memorable thing about it was how bad the special effects were. I think I'm wondering whether the plot makes any more sense if you watch the whole movie.
Anyway, there was this military-survivalist-type guy, a woman, and a teenage boy. The survivalist guy has this vehicle that looks like a cross between a camper van and a bullet; it's designed to withstand the imminent global flood that apparently only these three are expecting and no one else is aware of.
There was some weird name for the vehicle, but I forget what it was, "The __ _ _ __" I think it had four syllables, possibly ending in "er". They got into it on dry land, and the flood waters rose around them, apparently miles deep.
External shots of the vehicle showed it tumbling end over end, but internal shots just showed some mild jostling and the three of them just standing there looking bored. Maybe it had artificial gravity?
Anyway, the miles-deep flood waters recede (the impression I got was that in-world, the flood lasted maybe fifteen minutes, if that.) They find themselves on dry land, I think on some sort of college campus, with all the buildings intact.
They pick up some music on the radio, and then the (young-female-sounding) DJ comes on, gives her location, and then says she's pausing the broadcast to allow for incoming signals from other survivors. It's all very matter-of-fact, like civilization-destroying flash-floods happen all the time. She apparently safely rode out the flood in the college's radio studio.
Military-guy tries to take charge of the situation, and responds to the radio-signal, but teenage-boy is very excited to hear the woman's voice and jumps on a bike and heads off to meet her as soon as she gives her location. Roll credits.
I don't think survivalist-guy and teenage-boy got along very well, but I didn't see enough of their interaction to say for sure.
I saw (the ending of it) on television in either the late eighties or the early nineties. I don't recall there being anything that indicated for sure whether it was a newish movie or an older one, so I'm not sure when it was made.