The dense, bony structure and natural, gradual taper of a bill made it an exceptionally strong material for separating the thick strands of hemp or manila rope when splicing lines or untying hardened knots.
Here are a few examples of what these biological tools looked like:
Antique sailor-carved marlin spike. Source: eBay
Because these were custom-made by the sailors themselves (a form of maritime folk art known as scrimshaw or sailor's work), the handles were often personalized. Some were intricately carved with grooves for better grip, while others were tightly wrapped with tarred twine or leather to create a comfortable handle.
Cord-wrapped bill with a custom sheath. Source: Etsy
An interesting piece of nautical history: You might assume the tool was named after the fish, but it is actually the other way around. The tool was originally called a "marling spike" (from the practice of "marling" or binding ropes together). When sailors encountered large, bill-nosed fish, they thought the animal's snout looked exactly like their rigging tool, so they named the fish the "marlin."
