
I remember doing a calculation of normal water intake (wanted to know what size bowl I was safe leaving five dogs with for a couple of hours) and I worked out that on dry food my 20kg dogs should drink a litre each.
So your 30kg bitch should be having 1.5 litres normally, so 2 litres in pregnancy seems perfectly normal to me.
Of course she will need to pee in the night, it is probably like in people (remember it well) that the urine production continues unabated night and day, where in non pregnant state less is produced at night.