WordNet sense 1 (sinking until covered completely with water):

submergence, submersion, submerging

WordNet sense 2 ((astronomy) the disappearance of a celestial body prior to an eclipse):

ingress

WordNet sense 3 (complete attention; intense mental effort):

absorption, concentration, engrossment

WordNet sense 4 (the act of wetting something by submerging it):

dousing, ducking