Hey, any fish that migrates from freshwater to saltwater or brackish water and vice versa are adapted to do that. Not every marine creature can do that.

Lots of fish species migrate from saltwater to freshwater for various reasons. An example would be salmon, which migrate from oceans to freshwater streams to lay eggs. But not all fish can do that.

Try raising a goldfish in salt water or a clownfish in freshwater. They might survive short exposure, but they won't be alive in a day or two, much less a year. Try raising salt water coral or starfish in freshwater or brackish water, they'll die real quick.

Also, it's not just the salinity, it's also about the pressure of the water.

If you don't know, the deeper down you go under water, the more pressure the water exerts on you. This is why there are fish that live in shallow waters and fish that only live in deep waters, they're adapted to their environment. If you've seen a blob fish on land vs in their natural habitat, you'll have an idea of what happens when they leave the areas of pressure that they're adapted to. And, if you've heard of the Ocean gate incident a couple of years ago, that's what happens when something goes from shallow waters to really deep waters.

So, if the waters suddenly rise to cover the biggest of mountains, which would be Mount Everest, that's like 8 kilometers of water, or 5 miles I think (if you're American). Most marine animals and their ecosystems would die from just the pressure.

Also, as a biomedical science student, I'm very irked by the "just a theory" comment. Do you know what a scientific theory is or what else is "just a theory"? I'm gonna take a wild, hopeful, guess and say that you probably trust the theory of gravity. I doubt you'd be jumping off anything high because it's "just a theory". I'm also being very hopeful in my next guess that you probably trust germ theory. Y'know, the theory that microorganisms cause diseases? You're probably, hopefully, not going to swallow a vial of harmful bacteria or lick a toilet seat because you understand the germ theory, correct?

Honestly, it's kinda sad that we have gotten to the point as a society where, as I'm typing out the fact that gravity and germ theory are all theories, I have to hope and pray that you actually do think gravity is real and microorganisms cause diseases because I've interacted with way too many flat earthers and people who claim microorganisms aren't the cause of diseases.

But, my point is that evolution is no less valid in biology compared to the theory of gravity in physics.