Semaphore is a method of visual signaling used by the navy or even by people sailing with a yacht or a boat.  It is most commonly done by the use of flags but can also be done with lights.  Before the invention of the telegraph, signaling from high towers was used to transmit messages between distant points.  In the Navy it is used to signal between ships by sailors manipulating flags to indicate the different letters of the alphabet.

The flags used are the red and yellow Oscar flags.

And don’t forget:

bravo

zulu

BRAVO ZULU, or BZ, is one of the most puzzling expressions heard by non-initiates when dealing with Navy people.  It means «well done,» and comes from the two-flag signal.

If you are sailing Greece with a yacht it is very important to be familiar with the International Semaphore Flags as well as with the Phonetic Alphabet and Signal Flags