Hi Eleanor!
Absolutely, you can go to a Water Park and to DisneyQuest in the same day if you have the Water Parks, Fun, and More Option added to your Magic Your Way Tickets. The Water Parks, Fun, and More Options (Credits) are separate from your park tickets. For example if you have a 7 day Magic Your Way Ticket with the Water Parks, Fun, and More Option then you would have 7 Water Parks, Fun, and More Options (the only limit is the amount of days that you have purchased on your Magic Your Way Tickets which will correspond to the amount of Water Parks, Fun, and More Options that you will have). Each visit to a Water Park, ESPN Wide World of Sports Complex, or DisneyQuest uses up one of your Water Parks, Fun, and More Options (credits). I do this a lot when I am on vacation with my son. We will go to a water park in the morning, a theme park in the afternoon, and sometimes to DisneyQuest later on in the evening. So if you choose to go to a water park (Typhoon Lagoon or Blizzard Beach) and DisneyQuest in the same day, then you will be using two of your Water Parks, Fun, and More Options for each person in your party.
I hope that your next Walt Disney World vacation is truly magical in every way!
Keep Dreaming Big!
Marc