If fascinated by cave diving, then Swansea is the place to be. South Wales peninsula is famous for its caves as it combines dry caves with caves, partially filled with water.
Visit Brecon Beakons park with “Porth yr Ogof” cave with the biggest cave-entrance in UK. The cave-entrance sizes over 17 m/55 ft wide and almost 5 m/16 ft high. If you prefer a coastal experience, visit Gower and west of Swansea city where you would be exploring two Welsh caves, “Ogof Ffynnon Ddu” and “Dan-yr-Ogof”.
The first one is 308 m/1010 ft deep with beautiful river passages and abstract shaped stalactites. On the other hand, Dan-yr-Ogof offers classic UK cave routing. However, you need to be extra cautious since water level can be unexpectedly increased and thus deepening the spot without prior notification.
In any case you would need a leader instructor to explore cave diving spots safely. Many schools provide such services in the area.