Lost Kingdoms and Sunken Realms
I found some Welsh legends of sunken kingdoms. Considering the fact the country has a long stretch of coast with heavy tides, it should not come as suprise that such legends arose as result of floods. Moreover, the Welsh share a Celtic culture with the Bretons where such a legend is famous as well, that of Kêr Ys (Kêr/Caer, meaning fortress or stronghold).
Cantre'r Gwaelod is a legendary kingdom said to have occupied a tract of fertile land northwest of Aberystwyth, todays Cardigan Bay. Its capital was Caer Wyddno, seat of the ruler Gwyddno Garanhir, who in some legends is connected with Taliesin as grandfather or foster father.
Like Kêr Ys, the kingdom was protected from the sea by floodgates. One day the keeper of the sluice gates was drunk and failed to close them, with the result that the sea flooded the land. Some versions name the keeper as Seithenyn, and there is a story about him having been distracted by a woman, Mererid, who kept the keys to the sluices. Or maybe it was a fae responsible for the mess.
Gwyddno also held a landlocked portion of his kingdom to which he was able to flee, like King Gradlon of Kêr Ys. He was later called King of Ceredigion. The church bells of Cantre'r Gwaelod are said to ring out in times of danger, a legend shared with Vineta, a sunken city in the Baltic Sea.
The coast of Aberystwyth
Llys Helig was the palace of Prince Helig ap Glannawg who is said to have lived in the 6th century, and whose sons are connected with the establishment of several churches in the area. Helig owned an area of land between Llandudno and Conwy which was later inundated by the sea. Like Vineta's shadows in clear water, it is said that the remains of Llys Helig can be seen at low tides,
Some versions of the legend tell that the flood was the result of revenge because Helig's daughter Gwendud was unfaithful in love. In another version her lover Tathal treacherously murdered a Scottish chieftain to gain a gold torque and her hand, and the victim swore vengeance.