After thinking about the German diver knife and that used during the same time by the United States Navy (which in some respects shares a few features...notably a "beehive" pattern wooden grip), my thoughts are:
1. The US diver's knife is far more robust, double edged and acted as much a knife as a tool...to pry, chop or cut (with "stab" perhaps the least effective use).
2. The German knife seems more of a defensive weapon, with stab it's primary function and cutting a secondary one. As a "tool" it would have very limited use.
3. Since hard-hat divers were tied to an umbilical (for retrieval, air supply and communication), and sallied forth on their mission from either a dock/wharf, surface ship, or submarine, the last thing they would ever deal with was an enemy combatant. Dangerous marine life might be an issue, but a exceedingly rare encounter. The diver's real concern was moving around in conditions with poor visibility among sharp wreckage, tangled metal, wood, nets, ropes and steel cables. In this environment the USN pattern knife could prove to be a life-saver. The German...not so much.