Air pressure is about 10^5 Pa, I think, so that puts us somewhere down
   the bottom area of the liquid water phase.  The story looks about the
   same up to around 10^8 Pa, so we can consider this area first.
   I imagine it could be possible that excessive tightening of a threaded
   plug on a spherical enclosure or a steel water bottle could produce
   pressure in excess of 10^8 Pa, I haven't considered what might limit
   that.
   Moving our shaky finger leftward on the phase diagram, we may discover
   ice Ih, hexagonal ice, the most well-known ice.
   Ice densities are listed farther down the page.  In an enclosed
   environment, density will be unable to change.  So, I'm guessing that
   hexagonal ice won't be what happens.  I guess we'd get stuck on that
   phase transition line, as pressure rose.