Puree the watermelon in batches until smooth, then strain into a pitcher. Cover and refrigerate until very cold.
Bring 6 cups water to a boil in a medium saucepan. Remove from the heat, add the tea bags and honey and let steep 5 minutes. Add the mint leaves, cover and refrigerate until cold. Discard the mint leaves and add the tea to the watermelon juice. Serve over ice in tall glasses with mint sprigs, if desired.