The system is subject to some external pressure, usually atmospheric. Initially, the internal pressure may be taken equal to the external. As the volume starts to expand,the internal pressure will start dropping off very rapidly, because water's volume does not change much under pressure, and at same stage a cavity will form there. The cavity will not be completely evacuated, it will contain water vapor. The pressure on all the boundaries will never exceed the external (atmospheric) pressure.
If I understand your description correctly, what you are thinking about is basically a suction cup. It could be somewhat more efficient than a typical suction cup, but not in a major way.