Could aliens live under the ocean?

There is no scientific evidence that extraterrestrial beings live under Earth’s oceans.

It’s a popular idea in science fiction and in some UFO/UAP discussions because the ocean is vast and still not fully explored. While large parts of the seafloor have not been mapped in high detail, that does not provide evidence for aliens—it just means there are still many things we are learning about the ocean.

You may also hear about "transmedium" UAPs—reports of objects that seem to move between air and water. Those reports are part of UAP discussions, but they do not show that the objects are extraterrestrial.

From a biology standpoint, deep-sea life on Earth can seem almost alien: extremophiles live in crushing pressure, darkness, and around hydrothermal vents. Studying them helps astrobiologists think about how life might survive on worlds like Europa or Enceladus.

So the short answer is: maybe in fiction, but scientifically, there is currently no evidence that aliens live under the ocean.