Is it significant they're all near reasonable landing places? This either means that the prehistoric builders travelled by boat, or that you not went to ones that you could land near!
How different was the view of the world where the water was the main transport route? I see this on the Severn, right up to the last century everything was focussed on the river, now people have turned the other way. It's also interesting that the water was less of a barrier than it is now, with many more ferries and people paddling to and fro in their coracles. So in places you see two villages of the same name that face each other across the water, eg Purton, - presumably they were once considered the same place.