water is essential cause on ss especially, you are not eating and food does contain some water.
adding a little more to your shakes is really not noticable. you don't have to go as far as i do but an extra 50ml isn't much more.
i've got to the stage where i don't measure out my water for my shakes i can just tell by looking and if i make it up and it doesn't make it a pint i add more to my shake. it also helps that my jugs are all marked where my beaters go round and it helps tell me how much water to put it.
2 litres isn't too bad over the day. a black coffee is around half pint in a mug. extra in your shakes, half pint before each shake and you'll more than take in 2ltrs.