Remember that it is not the case that nothing happens below the boiling point and then everything changes at it. The dissociation of molecules from the liquid is happening all the time.
Liquid water has a temperature-dependent vapor pressure. And this pressure is still there below the boiling point. The water in your glass creates a fairly small pressure as molecules dissociate temporarily. But this pressure cannot overcome the external pressure of the atmosphere, so the water just "sits there".
As you raise the temperature of the liquid, the dissociation rate increases and so does the vapor pressure. In an open vessel at sea level, this increases until the vapor pressure equals atmospheric pressure and it can begin forming bubbles in the liquid.
When you do the same thing in a pressure vessel, nothing different is happening to the water or to the bonds. But the increased pressure prevents the vapor pressure from forming bubbles at that same temperature. Instead the temperature has to rise until the vapor pressure equals the actual pressure inside the vessel.