In a small saucepan sprinkle gelatin over water and let soften 1 minute. Heat gelatin mixture over moderate heat, stirring, until gelatin is dissolved. Add sugar and Port and simmer, stirring occasionally, 3 minutes. Remove pan from heat and stir in lemon juice. Cool Port gelatin to room temperature. Seed cantaloupe and cut into 1/4-inch-thick wedges. Discard rind and cut enough wedges crosswise into 1/4-inch pieces to measure 1 cup. Divide melon among four 3/4-cup ramekins or bowls and pour Port gelatin over it. Chill jellies, covered, until set, at least 6 hours, and up to 1 day.