Submersion is the act of being completely held under water (or liquid) for a long time. Scuba divers use breathing tanks to maintain submersion during long, deep dives.
Words with -merse or -merge come from a Latin verb meaning to dip, soak or plunge. How risky is that plunge? It’s a matter of degree, and you can find that highest degree of risk in the prefix, sub, under. Submersion is often death-defying. The legendary entertainer Harry Houdini thrilled his audiences by undergoing submersion in a 100-gallon milk can! (Don’t try that at home.)