I want to clear up what it means for a sac to be seen and for water to break during whelping.
The sac
All mammal fetuses have a placenta and are surrounded by a membrane, called the chorioamniotic membrane, or also known as the amniotic sac or just the sac.
With humans, the sac usually breaks before the baby is born and then eventually a baby is born.
With dogs, they are usually born in their sacs. They can be born without them, but it is more common for them to be born in them.
Each puppy has it's own sac and placenta. There is no overall sac for the litter; the only overall "wrapping" for the entire litter is the uterus. The uterus should remain inside the dog.
Water breaking
There can be an appearance of water breaking at the beginning of whelping for two reason:
If a sac breaks before the puppy is born, the fluid will come out and that is the dam's water breaking.
The dam may have a gush of lubricating fluid before the first puppy appears or the sac may break for the first puppy appears.
Either can appear to be "water breaking." With a little experience, you'll be begin to tell the difference between some lubricating fluid coming from the cervix or amniotic fluid from a broken sac.
If it's the lubricating fluid, then there's not a time issue.
But sometimes you'll see a sac or a puppy part along with the fluid. This means a puppy was in the vaginal canal.
If you see signs of a puppy like this, then you need to start your clock for that first puppy. If the puppy doesn't come out within an hour, you need to see a vet asap. Seeing the sac or a puppy part (leg, head, tail, etc) and no puppy comes within an hour, that indicates either a stuck puppy or uterine inertia and is a possible emergency.
If you want more detailed info about litter whelping/deliveries, what to do for stuck puppies, how to recognize an emergency before it's too late, how to handle umbilical cords and umbilical cord emergencies, check out Whelping Basics.