Asten,
Thanks for asking your question to planDisney!
The water parks,
Disney's Typhoon Lagoon Water Park and
Disney's Blizzard Beach Water Park, are so much fun. I can never decide which one is my favorite!
The Theme Park Reservation program using the
Disney Park Pass System is, as you suggest, just for the four theme parks. You do not have to make a park reservation for a water park. But you would need to make a theme park reservation for whatever theme park you wanted to visit
after you are done at the water park that same day. Otherwise, you would not be able to get in. Arriving later in the day to your theme park reservation will not be a problem!
BONUS TIP #1You cannot use a standard theme park pass to get into a Water Park. You have to pay for that separately, or have it be part of a
Water Park ticket option.
BONUS TIP #2If you would like to plus up one of your water park visits, I strongly recommend you consider booking either a
Beachcomber Shack or make a
Typhoon Lagoon Umbrellas reservation. In both cases you get a "home base" where you can keep your belongings and give your family a place to meet up. In the case of the Beachcomber shacks, you also get a nice, secluded place with incredible views of the wave pool for your family to connect and relax between activities. Just call (407) WDW-PLAY or (407) 939-7529 to make a reservation.
Write me back if you have any follow-up questions.
Have a Magical Vacation!Doug