Much of what we know about the interiors of the planets comes from the combination of the observations we CAN make and mathematical modeling of the things we CAN'T observe directly. We know, for example, how much the whole planet weighs by its effect on its moons and on passing spacecraft. By comparing that weight with the size we observe, we can determine that the average density (weight per cubic inch) of Uranus is just slightly above the density of water. Since water is very common in the solar system, the natural conclusion is that much of the interior of Uranus (and Neptune) are water.
We also know from direct observation of Uranus that the temperature near the cloud tops is about 57 K (-357 F) and that the temperature increases with depth at a highly predictable rate. From that, we calculate that the temperature near the center of Uranus is about 5000 K (almost 9000 F). Obviously, liquid water cannot get that hot without boiling away, unless it is under very high pressure. That is precisely what happens in the interior of Uranus and Neptune. The water there would normally be a vapor (gas) rather than liquid, but because it is under such high pressure and at such a high temperature, it is in a state in which gases and liquids cannot be distinguished. We sometimes call that condition a "superheated" liquid, and in the case of Uranus it probably extends over most of the interior of the planet. Beneath it there may be an approximately Earth-sized core of molten heavier materials, and above it is the gaseous atmosphere of the planet, composed mainly of hydrogen and helium.
Incidentally, the superheated water in Uranus is likely a mixture of water, ammonia, and methane, which are the "icy" materials found in all the giant planets of the solar system.
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