Winner of Computer Game Review's Golden Triad Award.
From the user manual:
U-BOAT is a campaign game consisting of a series of sea patrols during the opening hostilities of WWII. Patrols begin in the Baltic Sea, extend into the North Sea and North Atlantic Ocean, and finally end somewhere in the Eastern reaches of the Mediterranean.
A patrol starts when you leave a U-boat base, and ends when you return to any U-boat base.
How long you spend at sea on an individual patrol is up to you. You are limited only by your nerve and fuel reserves, and may put into a U-boat base at any time.