Hi Alyssa,
If you are only planning on visiting a Water Park for one day, the most economical thing to do is to purchase a Water Park Single Day Ticket.
These can be purchased on arrival at Disney's Typhoon Lagoon or Disney's Blizzard Beach and currently cost $49.00 plus sales tax for Ages 10 and over or $41.00 plus sales tax for Ages 3-9. (See
HERE)
Your other alternative would be to add the Water Park Fun & More option to your 3 Day Magic Your Way base tickets which would cost an additional $55.00 plus sales tax for all guests Ages 3 and over.
This would entitle you to three separate admissions to the Water Parks, DisneyQuest® Indoor Interactive
Theme Park, ESPN Wide World of Sport; Complex or a round of golf at
Disney's Oak Trail Golf Course, a relaxed, nine-hole walking course
designed for the entire family.
Wishing you a magical trip!