Not all of the ice is water ice. Some is methane ice, carbon monoxide ice, etc. That being said, I've definitely heard members of the New Horizons team speculating about water ice being responsible for some of the mountains they've seen and even the idea of Pluto having an internal ocean at some point.

To expand on the internal ocean point:

One of the big problems with Pluto is that we would not expect it to still be active. One of the hypothesis that has been proposed is that the heat from Pluto's formation is trapped in an internal ocean and is slowly being released as the ocean freezes.

Of course, even if this isn't what is powering Pluto, the other mechanism might still make the core hot enough for liquid water.

They mentioned that methane ice and CO ice are not strong enough to make mountains out of, but that water ice at those temperatures is.

Supposedly water ice is the most common ice in the universe though.

Water is plentiful in our solar system. Much of it is locked up as ice, but it appears there is lots of it in liquid form underneath the ice in several moons as well.

It's just oxidized hydrogen.

Hydrogen is the most common element in the universe. Oxygen is the third most common.