To make lime cups: Cut limes in half lengthwise; cut around pulp of each half with sharp knife, leaving peel intact. Scoop out pulp, using spoon to loosen pulp from peel, and reserve 2 tablespoons lime juice for granita. Set lime cups aside.

Stir together sugar and water in small saucepan; heat to boiling. Cool slightly.

Place watermelon in container of food processor; pulse to puree watermelon.

Place colander over bowl; pour pureed watermelon into colander to strain out seeds, forcing watermelon through with back of spoon, if needed.

Stir reserved lime juice and cooled sugar mixture into pureed watermelon.

Pour into 13x9x2-inch pan; freeze until firm, about 4 hours.

To serve, scrape frozen watermelon mixture with spoon to make granita.

Stir in currants for seeds. Mound granita in lime cups; serve on bed of crushed ice.