These two just love letting themselves sink in quicksand! They know how to get out, of course, so they just relax under the surface for as long as their lungs will let them, which gets just a little longer each time. This time around, they've decided to blow bubbles, letting a significant amount of their air float away. They plan on continuing until they're out of air entirely, but don't worry, they'll still be able to surface.
Don't mind that the charmander's tail fire is out, my headcanon is that changes to its state don't actually have any effect on their well-being, because, you know, just imagine something being so delicate that touching liquid in the wrong way immediately kills it. That's kind of stupid.