Sprinkle gelatin over water in a shallow, heatproof bowl. Let stand until softened, about 5 minutes. Set bowl over a saucepan of simmering water, and stir until gelatin dissolves, 2 to 3 minutes. Stir in mint, remove from heat, and let stand for 10 minutes.
Remove mint; holding stems over bowl, squeeze liquid from leaves into gelatin mixture. Discard mint. Stir in creme de cassis, and transfer mixture to an 8-inch square cake pan. Freeze until set, about 30 minutes, then refrigerate for 15 minutes (or overnight).
Spoon some of the gelee into the center of each melon half. Garnish with mint, and serve immediately.
Charentais melons, available at farmers' markets and some supermarkets, have striped gray-green rinds and orange flesh. Choose small ones that give off a honeyed perfume.