Hey, I've just started slim & save as well. From reading the programme, you're meant to have 4 packs a day (this can be all shakes if you want). If you just have three shakes, then you should be having a small low carb meal in the evening.
Re: water, this depends on your weight and height. If you put your statistics into the website it should tell you the minimum amount of water you should be drinking.
Hope that helps, and good luck!