For sailors, Most whistle/manuevering signals are morse code. Knowing Morse Code greatly assists one in learning these signals and their place in the overall scheme of things.

All commercial boats have a light connected to the whistle switch. Knowing Morse Code via visual means allows the mariner to "see" the whistle signal at night, thus knowing what he needs to do to avoid collision.

Many pilothouses now have air conditioners and their windows and doors are closed, making whistle signals hard to hear but easy to see at night.