Add the lemon juice to the water kefir and mix well.
Puree the raspberries and strain them into the water kefir mixture. If they are very soft, you can simply force them through the strainer.
Stir the mixture to fully incorporate the raspberry puree.
Place the mixture in a 9x13-inch Pyrex-style baking dish. Place the container in the freezer for about 30 minutes.
Ice will begin to form along the edges where the mixture meets the pan. Using a fork, stir the icy portions into the center of the pan. Repeat this process every 20 to 30 minutes until the mixture is frozen (generally about 3 to 4 hours).
Using a fork, scrape the frozen granita into flakey crystals. Cover tightly and freeze.
Can be served with fresh berries or a sprig of mint.