Combine the water and sugar in a small saucepan and cook over medium-high heat, stirring, until the mixture comes to a boil and the sugar has dissolved. Let cool to room temperature. Halve the avocados in half lengthwise. Remove the pit and scoop the flesh into a blender, along with the cooled syrup, and a pinch of salt. Blend until smooth, scraping the sides as needed. Add the lemon juice and blend just until combined. Divide the mixture among the molds, snap on the lid, and freeze until solid, about 4 hours.