Rinse lemon with hot water and rub dry. Squeeze half the lemon. Cut the rest into thin slices.
Put lemon juice, sugar and wine into a bowl or large glass (approx. 4 l capacity). Leave to infuse for at least 30 minutes.
In the meantime, cut the Charentais and watermelon with skin crosswise into slices about 1 cm thick. Cut out small hearts from the fruit flesh. Wash lemon balm, shake dry. Divide into glasses.
To serve, place melon hearts and lemon slices in the bowl. Pour on chilled sparkling wine and mineral water.