You can use ½ - 1 cups water in the recipe, if using full cream dairy or coconut milk, I would probably recommend only ½ a cup – it will be quite rich otherwise.
Choose a sweetener which is quite mild in flavour here so that it does not overpower the flavour of the polenta or lemon. You just want it to sweeten the dish, not add flavour. The most obvious choice here would be white sugar, but for a more natural sweetener, I use either a light raw honey, or coconut nectar.