I don't know if there are mermaids...
but if there are they'd not look like traditional mermaids.

If there is something of human intelligence or near human intelligence that we have not been disclosed about, then it is probably using camouflage, like the octupus.

I would imagine that if mermaids existed and someone managed to capture the image of it, they would be repelled, disgusted or frightened by it.

It would not look like a beautiful busty human woman or a muscular human man.

It is hard to imagine why an underwater creature would need hands with fingers anyway. Some fish do have a sort of hand/fin though. Tentacles can function pretty well for an octopus. It can open jars with lids.

I imagine that an undersea creature would probably have tentacles instead of hands, if it needed to manipulate objects.

A rounded human like head wouldn't really make sense either. It would probably be more stream-lined than rounded. Even a bottlenose dolphin or one of those bulbous headed whales still have a bit of a snout.

Really, I think we need to rethink what a mermaid would look like.

We know whales and dolphins are mammals and yet we don't think of them as like one of us. We don't imagine that they are like humans because they don't look like a human. But I don't think a human shape underwater would even make sense.
If you don't need to manipulate objects with hands in order to feed yourself and swim, then why have hands? Why not have fins? If you want speed, then you want to be stream-lined. You'd have at least a snout nose to cut through the water with. Your feet would be useless. You'd want to swim I think, not walk and balance with toes. You would want a tail of some sort. At least something like a seals flippers.

If you want to imagine what a successful water mammal would look like, look at what we already have. Seals, dolphins, whales and such.