All Aboard!
That warning call, often accompanied by the ship's whistle, gave notice that the vessel was about to get under way, and that any lingering passengers had better hurry across the gangway.
The boards of a ship are the planking, timbers cut long and flat, and usually no more that two or three inches thick. Unless you have been to sea, you may have trouble sensing the special meaning of those planks or boards. On one side is a dry bunk, food, safety and comfort. On the other side of those boards is cold water, and death by drowning. With that in mind, perhaps you can begin to comprehend the special importance of the word.
In the early development of the term, you will find the word "bord" which carried the sense of a line of demarcation, or a border. That word conveyed a meaning so closely related to the sense of the term "board" on a ship, that the two words just blended together in the English language.
You will find "board" used often on ships and boats. If you type the word "board" into the search field of the Seatalk Nautical dictionary, you will find 45 entries for the word. Most of them refer to that concept of planking, but there are many others referring to parts of a vessel that are made with thin, flat pieces of wood, such as trailerboards, centerboard and running boards.
The word is also commonly used to indicate a course sailed on a single tack. The expression "short boards" suggests frequent tacks.
"By the boards" describes losing something over the side of the ship, a term related to "overboard". A "boarding party" is a group of armed seamen who have the task of entering another ship by climbing over the side.
"Inboard" can be used to describe anything, such as an engine, that is mounted within the ship, while "outboard" describes anything mounted on the wet side of the planking.

