Cut a 1/2-inch slice off both ends of the prickly pears, then make a 1/2-inch deep incision down one side, end to end. Carefully (there are spines) peel off the rind, starting from your incision. If the prickly pear is ripe, the rind will easily peel away. Roughly chop the peeled fruit.
Place the fruit in a blender or food processor. Add water, sugar and lime juice and blend for several minutes, or until the sugar is dissolved.
Strain through a medium-mesh sieve. Taste for sweetness, adding more sugar or lime juice if necessary. Chill thoroughly.
Pour chilled mixture into canister of icecream freezer and freeze according to manufacturer's directions. When the ice comes out of the machine, it may be rather soft; for a firmer texture, let it ``ripen'' in your freezer for a couple of hours before serving.