To go back to the Welsh sites, especially Pembs, all along the coasts are small promontory forts,/fortified settlements; the thing that you notice is that they are all near caves that could have been occupied in the mesolithic period, and some of the portal dolmens are very cavelike in appearance, especially those that are near to the ground.
Been reading an interesting book - The Sun in the Cave, which more or less argues that early mythology stems from the rebirth of the sun from within the cave, or indeed from the sea - the going down of the sun on the west coast is very striking. Rivers are also seen as the source of renewal, that is why you have the great complex round Avebury, with the 6 streams converging and reappearing from the chalk. Practical people will argue that water is a point of necessity, for fish etc, and the same could be said of coastal settlements, with farming taking a secondary role.

p.s. There is an interesting set of neolithic tombs set very near the sea, which no one seems to have visited yet, its called The Morfa Bycham Cemetery SM2207, etc, hidden in the rocky landscape Neolithic Sites of Pembs,Cards And Carms.. Geo.Children and Geo Nash..