Jeju Water World is a family-oriented water park near Seogwipo World Cup Stadium, with a large indoor section and a smaller outdoor area. It is not one of Jeju's essential sights, but it is a useful rainy-day or kid-friendly option when beaches and waterfalls are off the plan.
The attached sauna and jjimjilbang make it feel more Korean than a simple pool complex. Bring swim gear and allow enough time to use the baths as well as the slides.