Here's a crazy idea. The Germans have U-boats. Did anyone ever consider the possibility of trying to attack convoys by approaching a vessel underwater and boarding it from underneath, blasting a hole in the side of the ship and entering that way? It sounds ridiculous, but who knows? The reward would be great if they pulled it off and managed to capture an enemy ship without anyone noticing. They can then surprise the rest of the convoy by having a friendly ship shoot at their own tankers and stuff.
And of course if they manage to blast a hole in the side of the target vessel and are not able to board, the U-boat can just bank off and let the target sink.