You have to pick:

In addition to simply being able to breath water/air life is also able to survive in its new environment. Humans can live in near freezing water without getting hypothermia, fish can fly through the air, etc...

You still need to be ready to get to your new environment at the moment of the switch.

Being able to swim through the air creates some interesting possibilities. Not only can we get dolphins as pets and use humpback whales eat mosquitoes for us we might also be able to harness this power for space exploration.

If sea life is immune to gravity (rather than just being less dense than air (which is also a possible explanation)) then we can grow fish cells in labs and use them to construct spacecraft. The structure of the craft will be weightless, so now we only have to get a few thousand pounds into orbit. It'll be a huge boon for space travel!

err.. flying Jaws, collossal squids and those frikken huge jellyfish? ahhh, land is my sanctuary.

I wouldn't go anywhere without an automatic shotgun in my hands and a turret mounted vulcan cannon mounted on the roof of my car. Fucking jellyfish man, pure evil and now they can fly!

So whales will just stay whales. And dolphins will just stay rapists of the sea. If having the live in the sea means all sharks either died or went to land okay.

But fish couldn't all grow limbs in ten years. Humans could swim, and we could get most of your pets to as well. But fish will just die because they wouldn't be able to fend for themselves.

Or! All fish including sharks will just live at the top of water and breath air when they needed to. Meaning we'd be eaten by sharks if we tried to surface. So nothing will live on the grounds. Except for maybe crabs and anything that has legs. So people and fish will live in water, afraid of sharks together and Crabs will grow giant and sentient overtime. And crabs will remember how we loved eating their ancestors limbs coated in butter. So will turn violent and start fishing for people.

Or! Humans somehow knowing this was coming. Will make bowls they wear on their heads so they can continue to live above water. Like when Sandy Cheeks took Spongebob to the surface out of the water. Though I'm sure people will figure out a more streamlined method of getting water near our necks/ears so we can breath. However people will choose to live in the water. Hippies and surfers mostly.

It would be odd, not to mention the DEEP dwellers of the ocean would never be able to adjust to the light.