Back on our moon, it seems there may have been a very heavy spurt of meteor bombardment at roughly the same time as the maria were extruded, suggesting that some of the heating may have resulted from those extra impacts. This is extremely difficult to verify after the fact. Even more recently, there is evidence that water exists under the icy surface of Ganymede, as well. The most likely answer is of course the simplest. Just as water exists at the base of Earth's glaciers because of the overlying pressure, the much thicker dusty ice surfaces of Europa and Ganymede probably exert considerable pressure on their bases as well.
High pressure results in temperature increases.
The Galilean moons are among the most massive moons in the solar system, so there will be some radioactive isotopes still present. Forming in the outer parts of the solar system also meant that these moons have a proportionately lower amount of metals and a higher amount of non-radioactive ices, so at best this heat source is a small contributor. Not like any ocean here on earth, of course. No crashing waves, no winds, no fish - and no light. But in one respect they must be similar, and that is in their salinity.