Water apples are an underwater fruit grown from a water apple tree and are edible for mer.
Water apples are described as being blue in colour, with a plump surface and a crunchy texture.
On the morning of the betrothal, Serafina's breakfast included water apples.
Becca bought four water apples from the market. When she laid them out in front of Astrid, she was embarrassed that that was all she had to contribute to their meal, as well as the crab eggs. Astrid claimed water apples were her favourite, though Becca doubted this was true.