Since we're talking about water ice here, wouldn't it be past the critical point at that temperature? In other words, no matter how much pressure is exerted on it the phase will always be a mixture of gas and liquid.
http://en.wikipedia.org/wiki/Image:Water_phase_diagram.svg
On another note, I think it's very unlikely to find a planet that is entirely water. Doesn't the current model for the accretion of the planets deal with cometary/asteroid impacts? In order to have a fully water planet with no rocky core not even a comet (dirty iceball) could have ever impacted it.
Yuk.
