Aqua Kingdom, as its name describes, is a kingdom mostly covered by water. Because of this, there's very little land to stand on. Coconut trees are common in this kingdom. Island-to-island navigation is often handled by bridges if residents aren't forced to swim. There may be some tall hills on the islands, but most of the land is beach-like.
This kingdom's underwater section contains passages that contain more of the kingdom. It is also the only way to access the more mountainous region of the kingdom. Pools of water can still be found in said region, though, necessitating the use of bridges and vines to navigate through the kingdom. The kingdom's castle can be found in this region. Just like the rest of the kingdom, the castle is even filled mostly with water despite the occasional (rather out-of-place) lava pools.
The ruler of this kingdom is a Giant Snake.