Thanks to Boise Ed for suggesting this.
The core for Ed of why this may be problematic is whether to take seriously the appearance of a body of water along the bottom border of the drawing, with therefore some air above it. If that casts the main picture as taking place in the air, well that sort of works for seeing the flying fish as actually performing sustained flight in the air. But it is something of a fail in terms of the human diver (with air bubbles!) and the other species of water-borne life — they can’t be understood as up in the air!
But of course that strip along the bottom doesn’t have to be taken as water below air. It could be just a decorative border. Or better, it could be the seafloor, below a water scene.
Comics I Don't Understand 
Yes, I see a narrow bit of seafloor, with an underwater scene above it, taking up the rest of the drawing.
“Flying” fish do leap out of the water for brief moments. But here they are swimming, fully in the water.
However, they do seem to have picked up the V-formation from their friends, the flying birds.
I’d say yes, the border is the sea bottom, not water. The flying fish are swimming, not flying. And the gag being that as flying fish they act like birds, moving in a bird-like formation even when swimming and going south for the winter.
Putting some rocks or plants jutting out of the bottom strip would’ve helped clarify the sea/sky issue.
Isn’t it established that the colorist is often disconnected from the artist? If so, this could be as simple as the colorist not getting it, making the ocean bottom blue instead of brown/grey/black.
Yeah, I figured it was a coloring error.
@ Phil & JJ (5 & 6) – It depends on the strip. When coloration is provided by the syndicate, it is usually simplistic “fill” or “panel overlay” design. Shading and watercolor effects (such as seen in this example) are a very strong indicator that the original artist (or a personal assistant) was doing the work.
P.S. When I first saw this strip, I toyed with the idea that maybe the scene was inverted (everyone is clearly in the water, but presenting it “umop-apisdn” made it look like they are flying in the air). The idea just doesn’t work (a coloring error on the sea bottom sediment is a much better solution), but only now did I notice that Tony Lopes is Australian. Perhaps being “Down Under” makes him think “umop-apisdn”.
As Kilby says, usually coloring for the dailies is a third-party service. Notably, when Breaking Cat News went to syndication, that’s what was used originally. Georgia Dunn decided to take over the duties because she wanted her watercolors.
One of the interesting things about Dick Tracy on GoComics (it also runs on Comic Kingdom, but I don’t read it there) is that the new artist, Charles Ettinger, sometime posts in the Comments. He has at times provided information about the daily coloring. In general, it’s better than it used to be, and he says that he provides notes for the colorist. He’s mentioned a few fails.
At one point, the strip introduced a new character in the Forensics department. For hair color, he said, “Do whatever you want.” Unfortunately, the colorist used something very close the regular character Lizz’s hair. This led some readers to assume it was her. The character really didn’t look all that much like Lizz nor was her hairstyle similar, but some don’t pay much attention.
Dvandom‘s idea is a good one. Ditto for pshiii. The bottom’s irregular edge and teal color still say “water” to me. I agree that if the larger blue and white area were water, it would all make more sense. The “flying fish” would simply be adopting a flying-wedge formation (now illegal in football).
“umop-apisdn”
Now there’s a name for a band…
Calling them “flying” fish just adds to the confusion.
That’s the basis of the joke. If they were regular fish, why would they be in a V-formation? Because they are “flying” fish, they are acting like birds.
Here’s a bonus Insanity Streak that is just unanswered questions for me. Are we supposed to catch a particular reference to “Young MacDonald”? Is this kid a character from elsewhere? What are those little spherical nuggets in the bowl, and do they spell something?
Umm, “Old MacDonald had a farm, E-I-E-I-O . . . . “
@ Brian in STL (11): But they’re not flying, so that feature is pointless. Just my opinion.
Brian, that occurred to me, but I didn’t see those letters. And the clearest bit is the word “it” on the longer pick, which also didn’t fit. Maybe from another part of the verses?
They aren’t flying, but they are swimming in a flying formation. Flying fish. If they weren’t flying fish, it would make no sense at all.
@ Mitch (12) – The bowl contains the eggs with which Y.M. is using to stock his farm. The only legible letters are “IT” on the rectangular note taped to the stack. Tony Lopes signs his own name in just such a rectangle; my guess is that “I.T.” was intended to give credit to someone who suggested the gag.
P.S. The IT turns out to be a Post-It seal of approval. Here’s Tony’s explanation: