Set aside 8 pineapple cubes and 8 melon cubes.
In batches in a blender, combine remaining pineapple, remaining melon, lime juice, honey, cayenne, and 4 cups water. Blend until very smooth.
Strain through a fine-mesh sieve, pressing on solids with a spatula to extract the juice.
To serve, run a lime wedge around the rim of tall drinking glasses. Dip the rim of each glass in chile-lime salt. Fill glasses with ice, then fill halfway with agua fresca. Top off with sparkling water. Garnish with pineapple and cantaloupe cubes.