Final answer:
Mariners should use the color of the top band and shape of the yellow mark on dual-purpose markings to navigate the Intracoastal Waterway; yellow squares for the port and yellow triangles for the starboard.
Step-by-step explanation:
When navigating the Intracoastal Waterway and encountering dual-purpose markers, mariners should be primarily guided by the color of the top band on the marker. This is because these markers are often used to indicate not only the lateral position in relation to the channel but also may provide information about the presence of the Intracoastal Waterway through a specific color-coding system.
In the case of the Intracoastal Waterway, most markers will carry a distinctive yellow mark or symbol in addition to the usual red or green coloration. It's the shape of this yellow mark that indicates to mariners whether they are following the Intracoastal Waterway. A square yellow mark generally indicates the marker is on the port (left) side of a vessel traveling the Intracoastal Waterway, while a yellow triangle indicates the marker is on the starboard (right) side.
Remembering this protocol assists in safe navigation and prevents confusion with markers that may serve a dual purpose, indicating both the Intracoastal Waterway and other waterways that intersect or run adjacent to it.