Check out the following phase diagram. A big ball of water would self gravitate to a sphere. The pressure would go as an exponential function of altitude. At room temperature, water will form "Ice VII" if the pressure is greater than 5 billion pascals (about 10,000 atm.). In theory, you could have a planet with a water vapor atmosphere, a liquid water ocean and an Ice VII solid core. However it is difficult to imagine a planet formation process that would produce only water and nothing else. As an aside, the action of exotic water ices has been proposed as a mechanism for forming some of the surface features on Mars.

Gary