Re: What would a planet made mostly of water (H2O) be like?

This is an extraordinarily difficult question to solve; the best I can do is give you some idea of what might happen.

The first difficulty is in creating a planet in the first place. During the formation of the solar system, the "ingredients" of each planet were set by the contents of the giant gas cloud from which the solar system formed, and by the cooling of the planets during that time. (Near the hot center of the cloud, the stuff which solidified was mostly rock; near the edges, more ice and hydrogen which condensed out.) I'm not sure that a typical solar nebula contains enough water to make a Jupiter-sized water planet... and if there was, it's almost certain that there would be huge amounts of hydrogen and helium collected as the planet formed, as well.

But let's leave that issue aside. Some things we can calculate fairly accurately: the mass of a Jupiter-sized planet made of water would be about 1.5 x 10^27 kilograms -- a bit less than the real Jupiter. (Jupiter is made of hydrogen, mostly highly compressed, plus some rock and metal.) Its surface gravity would be about 2 times Earth's. The pressure at the center of the planet would be roughly 700 gigapascals, which is 7 million times Earth's atmospheric pressure.

It would be useful to calculate the temperature at the center of the planet. The planet core will be hot due to energy released by radioactive decay, and due to the fact that most substances become warmer when you compress them. I am utterly unable to calculate this temperature. If the planet really is water all the way down, there won't be much radioactive heating... but there will be some. With a rocky core (which is very likely) there will be heating, leading to a temperature which might be as high as Jupiter's core temperature of 11,000 C.

We can do lab experiments to find out how water behaves for a given temperature and pressure. The results of experiments like this are shown in a phase diagram. For example, the diagram shows that at 100 kilopascals (1 Earth atmosphere) and 300 K (23 C) temperature, water will be liquid.

At the surface, the water planet's temperature will depend on how far it is from its star. If it's as far away as Jupiter, the temperature will be around 120 Kelvin (-150 C). The pressure will be essentially zero. It's clear from the phase diagram linked to above that the surface will be made of ordinary ice.

As we go deeper into the planet, the temperature and pressure will increase: plotted on the phase diagram, the pressure and temperature in the planet will form a curve tending up and to the right. The core pressure is very near the top of the diagram; the core temperature is far off to the right. Depending on how rapidly temperature increases, the curve may go through the area marked "liquid" on the diagram. This would represent a deep liquid "ocean" of water.

Eventually, the curve will pass into the region marked "VII". Water has many possible solid forms besides ordinary ice (Ice I on this diagram); each of these solid forms has a different crystal structure, and exists at different pressures and temperatures. The "VII" represents "Ice VII", an unusual solid which only exists at very high pressure, and which is stable even at temperatures far above 100 C!

I can't guarantee that ice VII continues to exist at the high temperatures likely to be found at the core of the water-planet (off the upper right corner of this diagram), but it seems likely.

So, the structure of the planet will probably be: a layer of ordinary ice on top, a layer of liquid water underneath, and a core of Ice VII at the bottom. It's possible that instead of a liquid water layer, there will be layers of Ice II, VI, and/or VIII, if the planet's core is relatively cool. It's also possible that another (probably solid) form of water will exist beneath the Ice VII layer.

References:
The Nine Plents
Windows to the Universe
Phase diagram of water
Lots more phase diagrams