So far, Earth is the only planet in the entire observed universe that we know, for sure, has stable oceans of water on the surface. Of course, we haven't really observed all that much, relatively speaking, and straight mathematics makes the odds of other Earth-like oceans pretty good. In fact, we're pretty certain that other celestial bodies in our own solar system have oceans. It's just that they're trapped under a thick sheet of ice.
for example, new research published in 2016 suggests that Enceladus, a small moon of Saturn, likely has a rocky core surrounded by an internal ocean that's entirely covered by kilometers of ice. Considering that Enceladus is 1.2 billion kilometers away, you might be wondering: How did we find this out? From its wobble, of course. Trace Dominguez has the story in today's DNews dispatch.
NASA: Cassini Finds Global Ocean in Saturn's Moon Enceladus
Space.com: Does Jupiter's Moon Europa Have a Subsurface Ocean? Here's What We Know
Seeker: Evidence Mounts for Ocean on Jupiter's Moon Europa