Combine water and sugar in a small saucepan. Bring to a boil. Stir until sugar dissolves. Turn off heat.
Crush mint leaves in a large bowl with mallet or spoon. Add to saucepan. Add lemon juice, orange juice, lemon zest, and orange zest to saucepan. Stir to combine.
Pass mixture through fine strainer, discard solids. Fill glasses with crushed ice and 3/4 full with strained mixture. Top with seltzer to cut sweetness (optional).