You may not need a compass on closed waters, depending on your navigation skill and the type of environment. River travel rarely requires the use of a compass as both banks are mostly visible and the flow of water provides a constant direction (downhill – which can be referred to on a map of the area).
On lakes, a compass can be handy when weather sets in or when traveling to areas where you aren’t familiar with the shoreline. It can be extremely difficult to find bays, creeks and even river mouths on highly convoluted and wooded shorelines as everything merges into a mass of trees and undulating ground.
Using a marine compass in conjunction with a map of the area allows you to find visible landmarks and plot a course using them in conjunction with your compass, ensuring that you travel in the right direction and find your way through the wilderness.