What naval strategy did Germany find most successful?
I am going to assume you are talking about the World Wars. If that's the case, then one has to point to Germany's use of submarines, which they called "U-boats"...
In both World War 1 and World War 2, the use of U-boats was a pesky, sometimes devastating strategy for the Germans to implement on the Allied forces of both wars. Submarines wrecked havoc on Allied blockades, brought down trade ships and battleships, and incited fear on the open seas. Indeed German U-boats were a terror, despite their lack of precision and frequent failure to commit to their mission.
An example of a submarine strategy that Germany used, is the "wolf pack" strategy in World War 2. It is basically what it sounds like; a group of U-boats surround and gradually attack a target. Another strategy, prevalent during both wars, was to to target trading and passenger ships. American trading ships were brought down during World War 1 by German U-boats.
Therefore, despite their overall precision, Germany's submarine warfare was formidable and a sizable threat to Allied efforts.