Hmm, I guess, sort of… The details in animal locomotion are really interesting and the gist of it is that there are different kinds of “swimming” and different kinds of “flying” and while some of these share similar mechanics, not all do.
As a quick example, there are some fish that power their swimming mostly with drag. With this kind of swimming they push their fins backwards on the power stroke, propelling themselves forwards by generating a lot of drag in the water. Then they need to retract their fins, and to minimize drag they might collapse the fin so it has a small area and produces less backwards thrust.
A duck’s foot works the same way. When pushing back the foot is splayed out, allowing the webbing to maximize drag. When retracting the foot it collapses down, to minimize drag. This kind of swimming is mechanically different from most forms of flying.
Contrast this with the fins of sharks that lay flat and have a single leading edge that cuts through the water. These fins work by creating lift and don’t get pushed and pulled through the water. This is also how penguin wings work and why it’s often said they “fly” through the water. So this kind of swimming is very much like flying.
In the air, wings generally need to generate lift to keep the animal airborne, while this isn’t strictly necessary under water. Also water is very dense, so many animals generate a good deal of thrust by undulating their bodies and rear fins. This isn’t generally very effective in air.
Here’s a good Wikipedia article about this kind of stuff: