This image was originally part of a slideshow on a website dedicated to St Andrew's University. It was taken down, but I retrieved it from the Wayback Machine. The original caption read: "The medieval stone pier at St. Andrews harbour that is largely built with stone from the ruined 14th century castle nearby 31st May 2004 11:09:36", so this was taken about six years after the events in Sons of Prophecy.

The dark object which just intrudes into the scene at centre right is the tip of a harbour bar: one of a V-shaped pair. The landward side of the bar is always deep water, aside from a small area of sand close against the bar at low tide. The far side, between this bar and the outer one and then on the far side of the outer bar, is shallow and is a wide, exposed sandy beach at low tide, but there is a narrow, deep channel alongside the pier which remains navigable at low tide, enabling boats still to come and go from the harbour.

This image from uniofstandrews.

You can see how high and narrow and exposed the distal 80-yard stretch is here: