Your humble hostess is quite certain that you understand "sailor speak" fairly well. Aft or stern is the back of the ship. Fore or bow is the front. If you are standing right aft (say on the quarterdeck of the lovely HMS Rose pictured above) and looking fore, to your right is starboard and to your left is port. So it has always been, yes? No.
Port came into common use in the British and American navies in the 1840s. Prior to that the word used was larboard. The word "port" as in "port your helm" is mentioned in writing as early as 1580 but on a day-to-day basis the usage was larboard and starboard.
The words may be derived from the Italian questa borda "this side" and quella borda "that side". These were then shortened to sta borda and la borda and then came to English as they now exist. That's just one theory but it makes sense to me. In French the terms are droit and de bâbord.
Obviously in a situation where a ship is being piloted through dangerous shoals, shallow water, etc. or in heavy weather the terms can easily be confused. A pilot calling out for the ship to larboard the helm could be misinterpreted, the helm is put hard to starboard and voila! Trouble on deck at the very least. A sunken wreck at worst. Not good.
In the Royal Navy, port became the official term in 1844. By 1846, the U.S. Navy had gone the same way. The distinction is certainly much easier but I miss the sing-song quality of the word larboard, especially when it's matched with starboard.
On another note, the division of crewmen into watches on an a ship of frigates size or smaller went by the names larboard and starboard. Sometimes the larboard men were known as larbolins.
There you have it, Brethren. Now you'll know that when that historical novelist includes a call of "port her helm" aboard ship in 1782, he or she hasn't done their research at all. At that point, it's time to find another book.