The claim which you read is that Ganymede has a 600 km deep outer mantel of salt water with a layer of ice over top of it. There are two reasons why they tell people that, i.e. they are trying to explain:

The ultralow moment of inertia, and:
The conductivity which they believe the require to explain the intrinsic magnetosphere while retaining the belief that such a magnetosphere would have to be created by an ongoing Dynamo effect.
Problems with this line of thinking include (minimally):

The fact that such a Dynamo effect would be ongoing and such an intrinsic magnetosphere created by a Dynamo effect (both on earth and on Ganymede) would be continuously topped off and would not diminish; nonetheless we know that our own magnetosphere is diminishing.
There are dark areas on the surface which you should assume to be solid land, and you cannot have solid land sitting on top of an ice sheet on top of a 600 km deep outer mantel of water, with or without salt.
A much more reasonable assumption is that the intrinsic magnetospheres both on earth and on Ganymede were fused in very early on by intense Birkeland currents and are basically remnants, having nothing to do with any sort of Dynamo effect.

The outer mantel of Ganymede is basically pumice and not water. Pumice is lighter than water and floats and would more readily explain the ultralow moment of inertia. Conductivity within such an outer mantel of pumice would be explained by the kinds of P-holes which creates surface conductivity on rocks. Pumice, of course, is by volume mostly surface, like the radiator in a car.

Putting Jupiter inside one AU of our sun (where you find most gas Giants and dwarf stars orbiting primaries within our galaxy) and Ganymede becomes a freshwater ocean world with light both from the dwarf star and the sun, and both bergs and anchored islands of pumice.