Much like history has recently found enough collaborating evidence to suggest [physical] unicorn legends -- especially the very old use of the term -- were referring to the white rhino, mermaids have long been known to likely be other now-known sea creatures, largely marine mammals. In fact, there is a journal entry by an early European explorer of North America about 'mermaids,' which is well-known to reference his first encounter with manatees.
It has long been the habit of humans to anthropomorphize animals, and the misunderstanding of things not seen up-close, or recalled from memory long after a far-off glance, has led to a lot of misconceptions which eventually became mythical creatures. Of course, we as a species also have a phenomenal imagination.
So physically, they do not exist.
Non-physically, who's to say? It's a matter of belief, and not fact. Some people may say they have encountered them as spirits or astral entities, while others may still insist otherwise.
Mermaids are essentially water fae. Water fae can come in many different forms but the appearance that is most often known is that of a mermaid. Water fae cannot be seen by the naked eye since they are a being that is a part of nature. In that sense they fall into the same category as angel and demon. Yet fae are very much alive. Mermaids have the ability to manipulate water as well as some controlling ice. They tend to be tied to environments and are known to be affected whenever an environment is messed with.
*They have also been known to sprout legs when on land
Mermaids I give a small sliver of hope there real because of how vast the ocean is, but I'm 99% certain they're not physical beings. I see them more as water spirits. I believe people can use them like archetypes or call on their energy in their craft [much like I believe you can work with vampires, werewolves, or whatever. If you feel a spiritual connection to something you do you. It's when people claim they're physically real and want to physically become one I get annoyed]