In a small saucepan over medium heat, combine water and sugar. Cook and stir until dissolved. Remove from heat and set aside to cool for about 45 minutes.
Place the cherries, lime zest, and lime juice in a blender or food processor. Blend until smooth. Add half of the cooled sugar syrup. Taste and adjust with more sugar syrup. Make sure the mixture is slightly sweeter than you want, because it will be less sweet once it is frozen. Strained the liquid through a sieve into a medium bowl. Discard the bits of cherry skins.
Transfer the liquid to a flat-bottomed 9 x 13-inch metal, glass or ceramic dish. Keep in the freezer and chill for 30 minutes. Cover with a plastic wrap.
Remove dish from the freezer, lift back the plastic wrap, stir the mixture throughly with a fork, cover with plastic wrap and freeze again for 30 minutes. Continue to freeze for another 4 hours, scraping and breaking the mixture with a fork every 30 - 45 minutes. All the liquid should be frozen into tiny ice flakes.
For the mascarpone cream (optional), in an electric mixer fitter with a whisk attachment, add heavy cream, powdered sugar, and vanilla extract. Whisk over high speed until soft peaks form. Add the mascarpone and whisk until combined, about 30 seconds.
To assemble, scoop granita into serving cups, top with a dollop of mascarpone cream if using. Serve immediately.