The idea that 'aliens' would live underwater in a more stable environment, the idea that humans aren't the first smart species on earth, and that the we keep hearing 'not from another planet'. Whales.

The silurian hypothesis is about another smart civilization on earth.

the crust surface under the sea is tectonically recycles more often then land crust

cetaceans as whole returned to the ocean somewhere like 50 million years ago

So there we have it. It's a bunch of species of futuristic super smart water mammals or whatever they evolved into.

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Somebody should make a movie about this.
- It's called The Abyss
"Smart" could mean a lot of things. Animals can be relatively smart. What makes humans special is out capacity for language. We have the musculature to create audible language, and the specialized neural lobes dedicated to creating and deciphering it.

Language, along with our advanced prefrontal cortex, creates the opportunity for knowledge transmission. I can tell my kids everything I've learned, and they can learn more and tell their kids.

Some animals have some of these abilities. Dolphins have large prefrontal cortices. Whales sing songs with tons of information we haven't even deciphered yet. Parrots can create language. Great Apes can be taught to understand language. Crows make and use tools, and teach other crows to do the same. But so far, we haven't found any animals that can pass down unlimited information the way humans do.

Interstellar space travel would require either super advanced technology or hyper-specific biological adaptations. The idea that whales showed up from another planet and became the whales we know today doesn't make a lot of sense in the revolutionary record. Now, if you said crustaceans or lichen or something were ancient aliens, that would feel more believable.
Whales don’t have hands. I feel that octopuses are more likely; they are already aliens.