Introduction: G: Primum miraculum est aliud m.e. in predicta regione uidelicet [The first wonder is another ?miracle. In the aforementioned region one may see the shore without the sea]
Sea: K, Durham, G, L: mare
It isn't especially clear where "the aforementioned region" refers to. The reference is from a British manuscript, which skip wonders 15-20. This would imply the wonder was in the same location as wonder 14, i.e. Ceredigion. There is actually a very nice raised beach around much of Cardigan Bay which is very well known and distinct. Under other circumstances it would be an obvious choice for an unlocated raised-beach wonder. However, manuscript K and its derivative L both headline this section with a de mirabilibus monie insule, suggesting Anglesey (Note). Although this doesn't appear to be the case for G (or, at least, it isn't mentioned in published sources), G was copied from the Durham manuscript, which was, in turn influenced by K. This may suggest the region referred to is "Mona".
Note that in at least one Irish manuscript Wonders 21 and 23 (which appear next to each other in the Irish versions, 22 being missing) are conflated into "A shore without a sea, i.e. a ford/shallow far from the sea"*.