Water apples go by many names, including wax apples and rose apples. I prefer the first one, because it describes the crisp, thirst-quenching quality of the fruit. The water apple is available in many colours, ranging from pale yellow-green to deep red. No matter what its colour, though, the fruit is bell-shaped and has glossy edible skin. The texture of the flesh is similar to that of the starfruit; it's not nearly as crunchy and firm as most varieties of apples. Also like the starfruit, the water apple isn't that flavourful - it's mildly sweet, but really quite bland. The fruit is prized because its high water content makes it refreshing.
The water apple is grown in tropical and semi-tropical climates, including many countries in Southeast Asia and East Asia (I've seen the tree growing in wild and urban areas of Hong Kong).
It can be eaten out of hand, like you would an ordinary apple, discarding the core and seeds. But to serve it more elegantly, cut it into wedges and remove the core. To perk up the flavour, serve wedges of the fruit with a dip: li hing wah mui powder (made from salty-sweet dried plum) or a mix of granulated sugar, salt, chilli powder and lime juice, as they'd serve in Southeast Asia.