water ice will float. SO both clog the lines

If water ice floats, wouldn’t it not clog the lines until you run out of propellant, in which case the clog is moot?

I was under impression that they pressurize tanks by pure oxygen

That was my impression as well. I wish I could figure out where people are getting the idea that they are using combustion exhaust to pressurize the tanks. Seems like a weird idea to me.