Scoop out cantaloupe and watermelon with a melon baller (yields about 8 cups total) and put in a 9-by-5-inch (8 cup) loaf pan; set aside.
In a medium saucepan, bring cider, water, and kanten to a boil. Reduce heat and simmer until the kanten is dissolved, about 5 minutes. Turn off heat and stir in honey until dissolved; cool to room temperature. Pour cooled liquid over the melon and set aside at room temperature until set, about 3 hours.
Invert the kanten onto a platter. Slice into individual portions and serve. Store leftovers in the refrigerator for up to two days.