One other theory that nobody talks about is, what if something was here millions of years before humans even existed. Maybe its still here.
There is also always the theory of interstellar or even intergalactic “portals” which might eliminate the largest obstacle to extraterrestrial visitors: the vast distances to travel to here from wherever. Some alleged UFO sightings have included what may be such portals, including at the infamous Skinwalker Ranch in Utah.
Intelligent life came first to our oceans ... If we’re sharing the planet with an unknown life form that’s a place to start.
From Plato to ancient Egyptian writings...
https://en.wikipedia.org/wiki/Sea_Peoples
The Sea Peoples are a purported seafaring confederation that attacked ancient Egypt and other regions of the East Mediterranean prior to and during the Late Bronze Age collapse (1200–900 BCE).[1][2] Following the creation of the concept in the nineteenth century, it became one of the most famous chapters of Egyptian history, given its connection with, in the words of Wilhelm Max Müller: “the most important questions of ethnography and the primitive history of classic nations”.[3][4] Their origins undocumented, the various Sea Peoples have been proposed to have originated from places that include western Asia Minor, the Aegean, the Mediterranean islands and Southern Europe.[