When the land sundered following the Gods War, the world was torn apart but not only throw the land but through the oceans as well. This, being a disaster of equal part magic and mysticism, did not just cause the oceans to all drain away into nothingness . . . though some did (such as the ocean that once existed around where the Gate City of Carcosa now is).
Only a small number of the oceans flowed away, their waters and inhabitants lost forever among the stars. Most of them just . . . stayed there, as if something held the waters back in a huge wall. Fish and other creatures can be seen swimming near to the water’s edge, but most know to keep from getting too close because whatever keeps the waters in does not prevent other things from moving in (or out) of the water. A few specialized fishers use small skyships or lowered platforms to harvest rare fish and other delicacies from the water walls but such is risky work.
A very small number of oceans just pour away into the space between the stars, impossibly never emptying, the sea water just pouring away but somehow eternally refilling. Needless to say, such places are incredibly dangerous both to ships on the surface and creatures under the sea.
Notes: Image “Osaka Aquarium” by kevin dooley is licensed under CC by 2.0.
Sea of Stars RPG Design Journal 