Shortly, Flash, Zarkov and Dale are captured under the sea by the minions of sexy Queen Undina (rhymes with...?). She's the ruler of Coralia, an underwater domed metropolis. Like all her gray-green-skinned people, she's a water-breather. Worse, Undina has used her laboratory to convert the Earthlings into water breathers too.
Zarkov and Dale attempt to reverse the process, while Flash distracts Undina on a swim to salvage their damaged rocket ship. Unfortunately, Dale and Zarkov are caught and made prisoners. They're taken to a dungeon, and Dale remarks "This planet seems to be filled with dungeons..." Clever observation, Dale.
Meanwhile, Flash learns from Queen Undina that she too hates Ming the Merciless, and that the despot would like nothing better than to rule Coralia as he does the rest of Mongo. "As long as Ming rules, there will be no peace on this planet," the Earthlings are told.
Before long, Ming sends his Gill-Men and an armada of submarines to attack Coralia. When Coralia's magneto-ray is destroyed by Ming's forces, all looks lost. However, Zarkov and Flash come up with a plan to super-heat the water around Coralia. They boil the water and destroy the Gill-Men, earning Flash and his friends the gratitude of Queen Undina.
Now, Flash Gordon has united another kingdom of Mongo behind his cause. Queen Undina agrees to convert the Earthlings back to being air-breathers and all's well that ends well. Thematically, as you might be able to tell, "Into the Water World" is a thematic retread of earlier Flash Gordon stories. Flash visits a bizarre kingdom of Mongo; sees it attacked by Ming; defends it, and gains a new ally. It's all overly familiar, and one has to wonder at the stupidity of Ming that he keeps letting his kingdoms slip through his kingdoms, whether it be Arborea, Vultan's city, or Undina's underwater domain. Also, I was sad to see Flash's cool rocketship get destroyed in a battle with a sea monster this week. I love that retro rocket design; and have an inflatable toy of it here in my office...