The Silent Enemy is based on Commander Crabb, a book by Marshall Pugh. This is the true story of young Lieutenant Crabb (Laurence Harvey), who in 1941 arrives in Gilbaltar to learn the rudiments of deep-sea diving. Crabb isn't interested in recreation, however; there's a war on, and it is common knowledge that a band of Italian frogmen have been sabotaging the British naval forces. Without official permission, Crabb and a band of hardy volunteers take on the task of scuttling the enemy's guerilla activities. Silent Enemy is at its best during its underwater sequences, in which both British and Italian frogmen deploy an astonishing variety of deep-sea weaponry.
~ Hal Erickson, All Movie Guide