After a couple billion years it can’t be ‘fresh’...............
Ganymede now joins Jupiter's Europa and two moons of Saturn, Titan and Enceladus, as moons with subsurface oceans—and good places to look for life. Ceres, the largest object in the asteroid belt, may also have a subsurface ocean. The new results come from Hubble's observations of Ganymede's magnetic field, which produces two auroral belts (pictured) that can be detected in the ultraviolet. Because of interactions with Jupiter's own magnetic field, these belts rock back and forth. However, there is a third magnetic field in the mix—one emanating from the electrically conductive, saltwater ocean and induced by Jupiter's field—that counterbalances Jupiter's field and reduces the rocking of the auroral belts. The Hubble study suggests that the ocean can be no deeper than 330 kilometers below the surface.
https://www.science.org/content/article/huge-ocean-confirmed-underneath-solar-system-s-largest-moon
I guess the explanation of the supposed “Tetragonal” layer of ice below the thick ocean is that under pressure, ice can form that is denser than salt water? But other analysis proposes that the mantle would heat water immediately above it. (I can imagine a liquid ocean above the mantle could be quite irregular and dynamic. Mountains of ice bumping about?)