This is for a fantasy world idea where the scale is meant to be way higher than earth. So I've been trying to make a term for the body of water that oceans would connect to, like how rivers connect to oceans.
So far, I'm working with modifying the word "pontus"
The word "ocean" comes from laying "oceanus", which came from Greek "okeanos". In Greek mythology, oceanus/okeanos is the titan of the sea. So it stands to reason that the term greater than okeanos would be the primordial god of the sea, pontus.
Not sure if it's a good full term on its own though, so I've been trying to find something to add onto it, or modify it the same way okeanos became ocean.
Edit: reading a lot of the comments, I should've clarified.
In the setting this takes place in, the premise was a fantasy world so enormous in scale it dwarfs planets, not just in the scale of its land or waters, but conflicts and life.
The reason there is a tier above ocean in this world is because each earth-sized section of continents the world was composed of already had a term for "ocean" prior to encountering the other sections. After meeting each other, a term was coined for the enormous and treacherous waters bordering each section. It's effectively an aquatic "wall" bordering each section.
"Thalassa" does sound good, though, so I'll probably leave with that one for now. Thanks for all current and future help