Re: Sub surface ocean? Yeah, right.
The highest pressure would be at the center. Ceres has a low density, so it's probably mostly ice. Radioactive materials are heavy, so any radioactive heating would have to come from a small, rocky core. If the core is rocky, it's not an ocean. Also, heating a planet by radioactivity depends on a smaller surface area to volume ratio to hold in the heat. Ceres is probably too old to still have any radioactive heating.
Ice under high pressure CAN be liquid down to around -20C (but at a higher pressure than calculated above). Surface temperature of Ceres is estimated in the range -70C to -140C. If the core is rocky any liquid water would have to be closer to the surface, and possibly under insufficient pressure to melt the ice even if there is still some core heating. Things are in the ballpark but the odds are small. See the following factoid:
The minimum temperature that liquid water can exist without ever freezing is -21.985 °C at 209.9 MPa; at higher pressures water freezes to ice-three, ice-five, ice-six or ice-seven at increasing temperatures.