>Burl's Megalithic Brittany has some small pictures of monuments that are under water at high tide due to climate change<

we saw those last year, if you are talking of the site I think you are. There is a circle on an island adjacent to the Gavrinis site in the Gulf of Morbihan. We didn't get a good view (could only see the tops of one or two stones), but from the pictures half the circle is submerged, the other half clinging to the side of the island. Lovely part of the world.

Squiddo
x

There are two portal tombs that reside in estuaries in south Ireland. One in Waterford, the other in Cork (I think)