I agree with JeanieK that she is probably getting water from the foods you are feeding her. Personally, I just give my nursing mommas a good quality dry puppy food (Chicken Soup and Canidae) mixed with some canned Pedigree Lamb and Rice puppy food. I just make sure mine have plenty of food available at all times. I especially make sure they eat at bedtime. I give them 1/2 of a Doc Roy's Healthy Bones tablet twice a day.
You can tell if the momma is getting enough fluids by looking at her urine. If she pees on a white pee pad and the urine is dark then that is a sign of dehydration. Keep in mind also, she is probably still stimulating the puppies to make them pee and she is getting that liquid also. If you feel she is not getting enough liquid then you could offer her some pedialyte.
Good luck with your babies.