Earth circa 250 million years ago

Well, Tethys was the goddess of fresh water in Greek mythology. She was also the wife of Okeanos (that's why she gets the ocean)

Interesting. Although aren't oceans salt water?

Yes, but fresh water flows into the them. The mixture of fresh and salt water deities is part of number of creation myths, including the Babylonian one. Though in that case, it's the reverse with Tiamat (girl, salt water) and Abzu (guy, fresh water)