Cut out hearts from the watermelon, discarding the rind. Try to space them as closely as possible to use as much of the fruit as you can. The discards can be used in smoothies or fruit salad.
Skewer each heart from the bottom, being careful not to push the skewer all the way through the fruit.
Using a mortar and pestle, combine the chile and salt. (If you don’t have a mortar and pestle, you can do this is a small bowl with a spoon, pressing the chile powder slightly to release its oils into the salt.)
Just before serving, sprinkle each heart with the chile mixture.
Squeeze 1/2 lime over each heart. Add additional lime to taste.