Complex sentence: He was pinned against the grating for some time by the pressure of the incoming water, until a blast of hot air from the depths of the ship erupted out of the ventilator and blew him to the surface.
Split into simpler sentences: He was pinned against the grating for some time by the pressure of the incoming water. A blast of hot air from the depths of the ship erupted out of the ventilator and blew him to the surface.