The Collaborative International Dictionary of English v.0.48:

Submergence \Sub*mer"gence\, n. [From L. submergens, p. pr.]
   The act of submerging, or the state of being submerged;
   submersion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

submergence
    n 1: sinking until covered completely with water [syn:
         submergence, submerging, submersion, immersion]