Hi, Monica!
Adding the Water Park Fun and More option to your ticket adds so much to your Walt Disney World vacation! You can absolutely do what you suggested. Start at a theme park (using 1 day of your park ticket) then head to a water park (using 1 of your "fun" visits). You are welcome to return to the same park, or if you also added the Park Hopper option, you can even switch theme parks with that same ticket that day.
When adding the Water Park Fun and More option, you get as many "fun" visits as you have days in the parks. This means that if you have a 5 day Magic Your Way ticket with Water Park option, you have 5 days in the parks plus those extra "fun" visits.
In addition to the Water Parks, you can
use your visits at DisneyQuest, Miniature Golf (before 4pm), visit ESPN Wide World of Sports, or even 9 holes of golf. So many options, so little time!
I hope you have a great time in Walt Disney World and get to visit one of the super fun Water Parks! Enjoy!