You were given these clues to the identity of National Park Mystery Spot 20:
They traveled this way during the fray, praying the wolf packs would fail to find them.
Whether it be wide or narrow, it's still the business end of the arrow.
A visitor center, yes, but no bridge to a key will you see.
The answer is Convoy Point at Biscayne National Park. Here's how you put it together:
During the World War II Battle of the Atlantic (1939-1945), German U-boats preyed on Allied shipping in the North Atlantic Ocean. For protection, Allied merchant ships crossing the North Atlantic were grouped together and provided with naval escorts to form a convoy. Groups of U-boats ("wolf packs") conducted coordinated attacks on the convoys.
The "business end" of an arrow is the pointed end -- that is, the point.
Biscayne National Park near Miami consists mostly of water and small islands called keys. However, part of the park lies on the mainland, and most of that mainland portion is at Convoy Point about nine miles east of Homestead. There are various facilities at Convoy Point, including the Dante Fascell Visitor Center. Islands can be seen across the bay, but they are accessible only by boat. Thus: "A visitor center, yes, but no bridge to a key will you see."