All we can say for sure is that no one has ever seen a real mermaid, but there is still A LOT we do not know about the creatures living in the ocean. In fact, the ocean covers 71% of Earth's surface and 94% of all living creatures on Earth live in the water! Scientists have only explored about 5% of the world's oceans so far, which means that we know more about the planet Mars then we do about our own ocean. There are many creatures living in the ocean right now scientists have not yet discovered. You can check out some of the cool things we do know about the ocean here:

In a word, no. Humans and fish are different animals and cannot be fused like that. Our closest relatives that live in the sea are the whales (including dolphins), and whales are not mermaids.