(found this on Reddit)
I found something in Steven and the Stevens. When Steven was going around into different periods of time, we see this . Notice how it is all sandy with a ship stuck in it, with no water anywhere.
And look at the description of Ocean Gem: the water mysteriously disappears. What if Steven traveled to this period of time where the episode takes place?
Again, just a reason on how the Crewniverse loves to hide stuff about future episodes in episodes.