A navigational aid, also called an aid to navigation, is anything that assists a mariner in the course of his passage on the sea. Today, there are many kinds of navigational aids, of which lighthouses are but one example. Indeed, it is the invention of new navigational aids that is threatening the very future of lighthouses, the electronic aid known as Global Positioning by Satellite (GPS), being the most obvious one. People often think that if they have a GPS device they don't need lighthouses any more. (This subject will be discussed elsewhere.)
It is very difficult for us, as we enter the twenty-first century, to appreciate the difficulties faced by seamen in the days before planned navigational aids. But going to sea is always potentially dangerous - even today. Think back to the earliest times in history when sailors had very primitive boats. In daylight, they had no way of knowing where they were, once out of sight of land. Not surprisingly, they tended to keep within sight of land, recognising geographical features on the shore to find their way. With experience or good information, it was possible to identify features of the land. The trained eye could also spot the swirling waters associated with hidden rocks. The vast majority of travel by sea was entirely local, sailors knew their own waters intimately and generally knew how to avoid most dangers. This idea of using features on the land that sailors could recognise was very useful indeed and gave rise to the idea of a beacon or daymark. (Note that a beacon does not necessarily have to be fire or light - only a visible feature.) Of course, some things were more readily recognised than others, especially buildings and other constructions. Churches and other religious buildings were often intended to be impressive and because they were large and often tall, they became features easily recognised from the sea. Eventually, tall structures were built that were specifically intended to be seen easily from the sea. We call these daymarks.
By night, sailors were better off in the open ocean where the stars could provide direction and they were far from the dangers of dark, rugged coastlines and submerged reefs. However, it was soon realised that lights shown from the shore could be used to help them navigate and to find their way back home.
'Swinging the lead', the process by which sailors checked the depth of water with a lead weight and a length of rope, was the only safe method of checking for submerged dangers and was usually employed only when danger was expected. The major civilisations such as the Greeks, Romans, Phoenicians and Vikings, which relied upon the sea to expand either their trade or their empires, travelled great distances in waters about which they knew little and soon realised the benefits of navigational aids - seamarks by day and lights by night.
For a complete description of the Origins of Navigational Aids, the reader is referred to the book: Pharology: Ancient Lighthouses by Ken Trethewey