Hi Crystal,
You worded your question perfectly!
Yes, you can use your water park pass on the third day. You can do both water parks that day, if you wanted to.
You could do the water parks each day if you wanted.
Now that my kids are a little older and can tough out a long day, we have started the day off at one of the
water parks and then hopped over to one park and then over to another.
If you are staying at a Walt Disney World Resort, take advantage of the
Extra Magic Hours, that way you can extend your day!
If you have any other questions, please let us know!
Wishing you a very magical vacation!!
Carrie