To make the simple syrup, combine the water and sugar in a small saucepan and heat until the sugar dissolves. Transfer the syrup to a container and refrigerate for at least 10 minutes.
Combine the watermelon chunks and 2 tbsp simple syrup in a blender; process on high speed until well blended, about 30 seconds. Taste and add more syrup if desired.
Pour the watermelon purée into popsicle molds, insert sticks and freeze until solid, about 4 hours. (To make chocolate "seeds," only fill the molds halfway and freeze; once solid, drop in a few chocolate chips and fill with the remaining watermelon mixture, then freeze again.)
Notes
Adjust the sweetness: Watermelon varies — taste the purée and add more simple syrup to suit.Chocolate "seeds": Freeze halfway, drop in chocolate chips, then top up for a cute seeded look.Plan for freezing: They need about 4 hours to set solid.Make extra syrup: Leftover simple syrup keeps in the fridge for other drinks and popsicles.