I'm definitely not an expert but I just wanted to give you my.opinions
1) I'm not exactly sure how much water a 12 week pup should drink. I keep a water bowl full at all times for both my dogs (a 15 month bulldog and a 15 week yorkie) I know for the bulldog I have to replenish at lest 3 times daily, but only once with the yorkie.
2) I'm really not trying to question you, but 7 hours? I don't know many humans that can hold out for that long! These pups can pee "a little" at a time and it would hardly be noticeable.
3) if she's not eating much(are you free feeding?)I suggest you put her in her pen(if you have one) with food and water and leave her alone for 30 mins to an hour so she can rest or eat if she's hungry. Sometimes puppies forget to eat/rest after playing for so long. Or she just may be a late night snacker. I've found with my pup that he eats the majority of his food at night when everyone else is sleeping.