When you think about it there are 3 different ways that you can communicate a message. Below is a picture that shows the Alphabet on the far left. Next is Signal Flags, Morse Code and then Semaphore Flags.
Signal Flags- The system of international maritime signal flags is one system of flag signals representing individual letters of the alphabet in signals to or from ships. It is a component of the International Code of Signals (ICS).
Morse Code- Morse code is a method of transmitting text information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment. The Morse code was developed so that operators could translate the indentations marked on the paper tape into text messages.
The following picture shows a picture of a Morse code machine.
Each character (letter or numeral) is represented by a unique sequence of dots and dashes. The duration of a dash is three times the duration of a dot. Each dot or dash is followed by a short silence, equal to the dot duration. Below is a video that illustrates that point with a flashing light. The video is long but you can watch as much as you want
Here is a close-up picture of the flashing light in the video. Note the hand on the lever, the shutters in the center. When the light is on you can manually open and close the shutters to communicate with your hand.
Semaphore Flags- Semaphore Flags is the telegraphy system conveying information at a distance by means of visual signals with hand-held flags, rods, disks, paddles, or occasionally bare or gloved hands. Information is encoded by the position of the flags; it is read when the flag is in a fixed position. Semaphores were adopted and widely used (with hand-held flags replacing the mechanical arms of shutter semaphores) in the maritime world in the 19th century. It is still used during underway replenishment at sea and is acceptable for emergency communication in daylight or, using lighted wands instead of flags, at night.
The picture below shows a US Navy crewman during a exercise.