This is probably more information than you want.
It sounds as if you need two Theme Park days and one water park entry.
What you would want is a two day base and buy a discounted one day water park ticket (with your DVC Membership Card) at the water park gate.
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 or Disney Quest tickets they cost $48.99 adult and $42.60 child. 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.
You might want to look at the Everything About WDW Tickets locked sticky in the TPA&S Forum. Especially the items bolded in the list below.
Post and Contents
1. This Table of Contents
2. Magic Your Way (MYW) Ticket Basics Including Definitions
3. Annual Passes and Water Park Tickets
4. Tickets Available Only to Florida Residents
5. Tickets Not Available to the General Public
6. Upgrading MYW Tickets (Not to Annual Passes)
7. Upgrading Tickets to Annual Passes
8. Dealing With Tickets Issued Prior to 1 January 2005 (Pre-MYW)
9. Children and Adults
10. Special Events
11. Comprehensive MYW Ticket Example
12. Hotel Card (Key To The World (KTTW))
13. Lost Tickets
14. Finger Scans
15. Frequently Asked Ticket Questions.
15A. Hopping Rules
15B. WPF&M With No Expire
15C. No Expire Upgrades
15D. Upgrading an Unused Ticket
15E. Upgrading Special Tickets
15F. Checking on Old Tickets
15G. Renewing Annual Passes
15H. Upgrading a Completely Used Ticket
15I. Getting Park Tickets Before Package Start Date
16. Resident 4 Day Wild 4 Disney Pass
17. 2010/2011 Armed Forces Salute Tickets
18. Prices Effective August 5, 2010
19. "Free" Tickets and Special Promotions
20. Ticket Purchase/Upgrade Locations and Telephones
21. Florida Resident Installment Purchases of Annual Passes
To get there directly, follow the link in my signature.
and I'm thinking of planning a Mommy and Me trip for July with my DS5. He got a Daddy and Me trip to DL in October, I figure it's only fair. 
