There is a longstanding tradition that Witches cannot cross water. Understanding why this is believed when it is clearly not true in a practical sense casts some light on the original practices of witches.
The roots of the tradition are seen clearly in works of fiction. The first of these is in the Robert Burns poem the Tam O’Shanter. In this a farmer passes by a church on the way home from a pub. He sees witches dancing but he is noticed and the witches chase after him as he rides his horse. Tam understands that if he can get to the bridge then the witches will not follow him. He gets to the bridge but one of the witches gets hold of the horses tail and pulls it off.
This idea is also referenced in the Harry Potter novels,
“Even though Petunia was raised alongside a witch, she is remarkably ignorant about magic. She and Vernon share a confused idea that they will somehow be able to squash the magic out of Harry, and in an attempt to throw off the letters that arrive from Hogwarts on Harry’s eleventh birthday, she and Vernon fall back on the old superstition that witches cannot cross water. As she had frequently seen Lily jump streams and run across stepping stones in their childhood, she ought not to have been surprised when Hagrid had no difficulty making his way over the stormy sea to the hut on the rock.”
The basis for this idea is that when witches cast circles they create a magic castle, often surrounded by four watchtowers. This castle is surrounded by water. To get to the castle the witches cross the river between the mundane world and the magic castle. In woodland working this is often shown by small brook, for example the workings of Robert Cochrane show this. Alternatively a broom can be used to show this, and witches can cross the broom to get into the castle. Once everyone is in the castle a circle is usually formed. Once the circle is formed it is not permitted for anyone to cross back into the mundane world. In this case Tam should have been right in that if he was spotted in the circle the Witches would have only followed him to the edge of the water. If he crossed it they would not follow.
The idea of forming a river around the edge of a magical assembly is fairly universal in Witchcraft and Wicca. In some traditions water and salt are mixed at the beginning of a rite and this is the representation of water at the edge of the circle.
The origin of this belief and practice is revealing. The ancient witchcraft of Britain is a fusion of two different traditions. The first of these is the shamanic practice of dancing the cloven hooved animal, often a deer as seen at Abbots Bromley or a horse. This tradition includes the myths and fairy tales of Britain, many of which include magic castles.
The second origin is the Babylonian Witchcraft tradition. In this four quarters are cast. These are based on the idea of the creation of the universe. This is recorded in the Old Testament, but is actually based on older Sumerian and Babylonian myths. In this prior to the creation of the world there was a salty ocean called ‘The Waters’ The creation of a magic circle involves the creation of these four elements. The spirit calms the water (water), creates light (fire) and then separates the firmament of the air (air) from the earth (earth).
In both of these the magic space is surrounded by water, and we can therefore see how the idea that witches would not cross water could have a basis in reality.