It's not known, at the current time, how thick the Europan ice shell is.
It could be a few tens of kilometers.
It could be hundreds of kilometers.
Recent evidence suggest there may have been
liquid water, very near the surface of Europa.
The fractures that we observe on the surface of
Europa are probably created by the force of Jupiter
pulling on Europa, it's vast gravitational force.
We would expect fractures to form a very predictable pattern
if they were just very basic geological features from ice cracking.
But they become increasingly different from what
we would expect, the older they are, and
this would make sense if the interior of
the planet was rotating faster than the exterior.
What that suggests therefore, is that the core
of Europa is detached from the icy surface, and
that might be by liquid water ocean.
There's other evidence for a water ocean as well.
Saltiness in the ocean perturbs the magnetic field of Jupiter, and this
is evidence that's been picked up
by studying Jupiter's magnetic field around Europa.
This talk of evidence also suggests that Europa has a salty ocean inside it.