The amount of water you will need to use to cover the can of sweetened condensed milk while cooking is going to vary depending on the size and shape of the slow cooker you use. Just add enough water to cover the can completely by at least 1/2 an inch. Because you are going to cook the can in the slow cooker you won't loose too much water to evaporation, but if you find that your can is no longer covered by water while cooking just go ahead and add more water so that the can is always covered by water.