Whisk together the water, milk, rice and vanilla in a medium saucepan and bring up to a gentle simmer on medium heat. Continue to cook, whisking often (a whisk works to loosen the rice if it is sticking to the bottom of the pot), until the rice is tender, about 15 minutes. Remove the pan from the heat and stir in the sugar and raisins, if adding. At this point the rice pudding may look watery, but it will thicken up as it cools. Transfer the pudding to a bowl to cool. Once cooled to room temperature, cover and chill completely, about 2 hours.
Before serving, whip the cream to a soft peak and gently fold this into the chilled rice pudding. Spoon the pudding into serving dishes and top with a sprinkling of the sliced almonds and ground cinnamon.
The rice pudding will keep, refrigerated, for up to 4 days.