What is found on the lead line at the 20 mark?

The presence of two knots tied to the line at the 20 mark aids in identifying the specific depth of water a vessel is navigating through. In nautical practice, lead lines are marked at certain intervals to help mariners ascertain how deep the water is at a glance. The knots serve as tactile indicators that can quickly inform a sailor of the water depth without needing to look at the markings on the line. This method enhances situational awareness and aids in navigation, especially in shallow waters where depth can quickly change.

Each option represents a different method of marking or signaling depth. While a blue ribbon, a single knot, or a white flag could be useful in various contexts, they do not serve the specific purpose of indicating the depth in the same manner as the two knots do. The two knots provide an essential and practical immediate reference for the crew while managing a vessel in potentially hazardous conditions.