We've all heard it so many times that it's just one of those things that's obviously true. But how many of us have actually seen a flying fish and watched what they do?
I've watched them many times, and I think that "obviously true" assessment doesn't do justice to the behavior and abilities of the flying fish. Watch them for a while, and you will see that they don't "just glide."
For one thing, they have considerable control of their direction and flight attitude. They can, to some extent, choose where they will reenter the water. They can steer around breaking whitecaps or floating debris. They can cut their flight short and descend sharply to the water when they want to.
More remarkably, they can, and in fact they do more often than not, gain power, airspeed, and altitude without reentering the water.
Well, without reentering the water completely.
The lower lobe of the tailfin of every flying fish I ever saw is noticeably larger than the upper lobe. After the fish has nearly lost its initial energy and begins approaching the water, it will usually arch its body to dip this lower fin back into the water, then it shuttles its tail back and forth to gain speed and rise into the air again.
If the water is really dead flat, they just dip their tails into the water wherever they happen to be when they get too low. However, if there is any swell at all, even the long swell in a ripple-free sea, they seem to choose to dip their tails into the water on the "uphill" side of a wave, and so get a "ski-jump" effect that maximizes their altitude.
They can do this repeatedly on a single "flight." I've seen at least five cycles of dip-shuttle-rise that carried a flying fish for more than two hundred yards without returning to the water. I suspect that the limiting factor is either their skin drying out, or their blood oxygen running low. In a pinch, I could imagine a flying fish extending a "flight" for over a half a mile if it needed to.
Now, this isn't "flying" the same as a bird or a bat or an insect flies, but I think it's a good deal more than "just gliding."
Epilogue: I've read that there is at least one species of flying fish that flaps its "wings" in flight, but I've never seen it. I've also read that at least some flying fish can use the "ridge lift" of a wave to extend their flight, much as an albatross or a pelican glides, but I can't say as I've seen that either. Nevertheless, I still believe that to say a flying fish "just glides" is understating the truth about this remarkable family of fish.