They would be destroyed in the ocean, I believe.
I like to picture them doing the cartoon thing where they run over open air (or water) for a while before figuring out that they have no ground and sinking.
The count as being destroyed in the Ocean. The Special Rule says[*] "You Drown any invaders or [Dahan] moved to those Oceans", meaning that they are Drowned after the movement, not before. (If there were an "instead" in there somewhere they would be drowned before the movement.)
You could target the Ocean with Vengeance of the Dead. Of course that would only be useful if you reach the threshold.
[*]This is the revised wording from the second and third printing, but the difference in wording is irrelevant for this point.