Warlords of Atlantis is a Doug McClure film from 1978. Cyd Charisse is in this! As is John Ratzenberger. I'd never heard of it before coming across it on youtube. It seems Atlantis' 7 cities are inhabited by invaders from Mars who got trapped under our sea with dinosaurs by accident. They aim for world domination so they can escape. It's all a bit confused, but mindless fun.
It's online at youtube in 10 sections. Part 1:
part 2, part 3, part 4, part 5, part 6, part 7, part 8, part 9, part 10
Moria has some positive words. Million Monkey Theater calls it "a fun romp".