"Wonder three: the shallows/ford that is there: when the sea is inundated also it is inundated, and when the sea decreases also it is reduced."1,2
Inland water bodies that act with the tide are consistently regarded as wonders across the ages3. The most common features involved are wells and springs (see, for example, Wonder 6) all the more remarkable when the water issuing from them is fresh4 (though we have no indication that this is the case here). While most translations of this wonder go with "ford", it seems unlikely why, for example, does it not mention the associated stream instead? "Shallows" seems more reasonable. As it is, there are no obvious records of tidal springs or wells or fords on Anglesey, though there are areas of coastal geology that would be appropriate. The nearest feature to the wonder-description is the lagoon Bae Cemlyn, which partly fills through percolation of water through the pebble bank between it and the sea* though it is not clear how old the shingle bank is5,6.