If you are only planning to have one water park or Disney Quest day you are much better off purchasing separate tickets.
If you add the Water Parks Fun and More option to your MYW ticket it will cost $57.51 (including tax) per person, adult or child. It will add one WPF&M entry per day on the base or hopper ticket (but two entries on a one day ticket).
If you purchase one day water park tickets they cost $48.99 adult and $42.60 child. (Disney Quest are $44.73/$38.34) If you are an Annual Passholder or DVC Member you can purchase one day water park tickets at an approximate 10% discount.
This upgrade may be done at any Park (including water parks or TTC) ticket booth or Guest Relations Office. If it is on your room key it can also be done at the Resort Lobby Concierge.
(Note each person must have their own ticket. Even though it adds multiple entries to your ticket, you can only use one ticket for one person.)
Also, if you are planning to do three or more water park visits within a year there is a definite price advantage to purchase Water Park Annual Passes.
We have two unusual policies at the water parks concerning ticketing. Both involve use of a one day water park ticket.
One is we will allow full value of the used WP ticket to upgrade to a WP AP as long as it is within 14 days of use.
The other, which is the only time combining tickets is allowed, is we will merge a used WP one day ticket with a MYW ticket to add the WPF&M option and only charge the difference in price.