Marinate the watermelon with the sugar and lemon juice for about 1 hour. Soften the gelatin in cold water.
Puree the watermelon and pour through a sieve and mix with the sherry or apple juice. Rinse a jelly mold with cold water. Dissolve the gelatin over low heat with a little bit of the watermelon puree. Add the remaining puree and stir over a cold water bath until the mixture begins to gel.
Pour the jelly into the mold and chill.
Shortly before serving, plunge the mold into warm water and flip the jelly onto a dessert plate. Serve garnished with fresh mint.