Dominick,
Thanks for asking your question to planDisney!
The Theme Park Reservation program using the
Disney Park Pass System is just for the four theme parks. You do not have to make a park reservation for a water park.
So, as you described in your question, you would still want to make a theme park reservation for that theme part that you wanted to go to AFTER your time in the water park, but you do not, and cannot, make a water park reservation. You just go!
The Park Hopper option that you bought as part of your theme park ticket package is just about going to more than 1 theme park in a day. So, in your case, you could go to a water park, then to a theme park, then to another theme park, all in that one day! That is a lot of fun in the sun!
BONUS TIP #1If you would like to plus up your water park visit, I strongly recommend you consider booking one of the reserved spaces. At
Disney's Typhoon Lagoon Water Park you can book either a
Beachcomber Shack or 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.
BONUS TIP #2If you end up visiting
Disney's Blizzard Beach Water Park, they have
Polar Patios and
Lodge Umbrellas. These can be reserved the same way as their counterparts in Typhoon Lagoon.
Write me back if this leads to any follow-up questions.
Have a Magical Vacation!
Doug