Watermelon Lemon Drop Slushies

diced watermelon, deseeded (freeze before blending for 1 hour)

shots lemon vodka (or your favorite flavored or unflavored vodka), optional

shots watermelon schnapps, optional

juice of 1 lemon, or to taste

sweetener, to taste (I recommend a liquid sweetener like agave, simple syrup, or a zero-calorie sweetener; granulated sugar may not dissolve entirely)