What's surprising to me is that Ganymede might have more water in that moon than all of earth's oceans.

Why? Because:

begin partial quote from:

https://en.wikipedia.org/wiki/Ocean#Extraterrestrial_oceans

The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water 

Here is a section on that same page called "Extraterrestrial oceans"

Extraterrestrial oceans

Main articles: Planetary oceanography, Extraterrestrial liquid water, and Ocean world

Further information: List of largest lakes and seas in the Solar System

Extraterrestrial oceans may be composed of water or other elements and compounds. The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, which are made of hydrocarbons instead of water. However, there is strong evidence for subsurface water oceans' existence elsewhere in the Solar System. The best-established candidates for subsurface water oceans in the Solar System are Jupiter's moons Europa, Ganymede, and Callisto; and Saturn's moons Enceladus and Titan.^[164]

Although Earth is the only known planet with large stable bodies of liquid water on its surface and the only one in the Solar System, other celestial bodies are thought to have large oceans.^[165] In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies.^[166][167]

Supercritical fluid on gas giants

The inner structure of gas giants remain poorly understood. Scientists suspect that, under extreme pressure, hydrogen would act as a supercritical fluid, hence the likelihood of "oceans" of liquid hydrogen deep in the interior of gas giants like Jupiter.^[168][169]

Oceans of liquid carbon have been hypothesized to exist on ice giants, notably Neptune and Uranus.^[170][171]