If we could breathe water we wouldn't have the same respiratory tract we would have gills. The way gills work is that it is part of the circulatory system with constant water moving over the capillaries (water in through mouth, out through gill flaps) and the capillaries engage in Weird Biological Stuff™️ and receives oxygen while expelling carbon dioxide. We likely would have no function for lungs (even though there are some fish who have lungs). So this means our anatomy probably would look different. We most likely would have a stronger jaw to keep our mouths open while breathing. We most likely would have our gills on our necks because that is where our carotid artery is and we would most likely receive oxygen there as it is one of the most significant arteries, sending blood directly into the brain. Even though I am fond of mermaids having their gills on their ribs. Apart from all of that, our eyes would also change because as of now, human eyes are not meant to withstand the pressure of water directly on them. Fish also don't have eyelids so perhaps we would evolve to not have eyelids. Also we have different shaped corneas than fish so they would also change probably.
The reason humans as they are cannot see air but can see water is because they are different densities. But when our eyes change so that we are better able to see underwater then perhaps we will be able to not see water but yes see air. Who knows?
But then you say what if humans could breathe both, are you referring to a specific animal that can do both? Because the things I am thinking are like, penguins, dolphins, whales, seals, and the thing with those critters is while they can hold their breath FREAKISHLY long. They can't breathe underwater. Or are you suggesting our body would morph to be something like a tadpole that way we keep most of the same physiology in the respiratory department but instead have outer gills which inhale through diffusion. But diffusion is when there is less molecules on the inside so the molecules wanna spread out and go inside. Because I hate to break it to you but our bodies would probably have to be a LOT smaller for that to work any sort of well. We're all gonna have COPD ☠️☠️☠️
Also while we might have some sort of olfactory sense we probably would not have noses the same way we do now because our noses currently connect to our respiratory system. Fishes don't do that ☝️ while fishes do have noses, it is different from their respiratory system and it just stimulates the nerve cells in their nose and their brains go "omg a scent" and yes because of our new human physiology, we probably would smell things differently than we do now.
We probably would not mind the taste of water because it is constantly streaming into our mouths so we can breathe so it would become like dead to the taste of water after a while. Which would be when you're a baby if this ever evolves into reality. I mean we might taste plankton or something if they ever decide to just float on into our mouths but other than that probably not
And you might be asking "if we as fish have to keep our mouth open all the time, how will we eat?" And let me tell you I had the same question. Luckily Britannica knows the answer. So imagine you're a happy little fish human and you're walking along your new atlantian home and you find a crab! What a delicious treat! So you bite him, in doing so you have water in your mouth because you live underwater in the Lost city of Atlantis of course because we're not cheapskates around here. Well actually we might be we just ate a crab off the street. Whatever so we're eating the crab and we're chewing well at the same time, the water that was in your mouth is flowing over your gills and out your gill flap as you swallow your bite of crab down your esophagus. So then after you swallow you open your mouth again and in comes more water at the same time you're taking another bite, you see where I'm headed with this.
I hope this helps your question. All of my research was done on Britannica. That was mostly for the tadpole section though because I'm decently good at marine biology but frogs aren't marine animals for like. The majority of their lives
Disclaimer: I am not a marine biologist I'm just a person who likes mermaids
You also might be interested in sirens because sirens are known for singing above water when they are mermaids and have gills so they can probably also breathe air if they are singing on rocks above water. Make sure you look up siren mermaid if you are interested in that because "siren" might give you like. Flashing lights or something idk. No photosensitive epileptic seizures or migraines on my watch ☝️ /silly /j