Information for each relevant Stone location is in the Stones by, in or near Water PDF. 19 of the 23 Stones have been identified with their original position near water.
Proximity to water can have several purposes including access for travel or moving materials, refreshment or spiritualistic purposes. Water is variously used, dependent on belief set, for baptism (partial or immersive), ritual washing, purification, connecting with others, creating relationships etc. Those Stones that are located by or near water have the name of the river etc. included. Maybe courses were different 1300+ years ago!
Any perceived “significance” for these Stones being close to water is considered in the context of religious belief and practice. For example baptism in Christianity and the proximity with water for Temples of Mithras. As such the specific locations reinforce their use as a gathering place for pursuing a religious belief.
For Mithraism this has been described by the author as an “Outdoor Mithraeum” whereby the Symbol Stone is the terrestrial part of the Mithraeum with its skyward view towards the planets and celestial sphere.
For Christianity the use of a Stone with a Cross would have served as an outside place of assembly for worship later progressing indoors to a church. Of the 23 identified locations above 16 are near churches