You can choose to go to a water park on the same day you go to a theme park. You can also choose to go to two different water parks on the same day.
Water Park Fun & More admissions are good for the water parks, DisneyQuest and a 9-hole round of golf. With a 10-day the,e park ticket, you have 10 admissions to the places I just named in the last sentence. You can use them up any way you want … more than one on the same day, one or more on the same day as you also use a theme park day, and so on.
So in other words, you don’t NEED to decide for any given day between a theme park OR a water park/DisneyQuest admission … you can do both if you like.
And as soundgod said, all the entitlements on your ticket are valid for 14 days from the day of first use. The day you first use a ticket is Day 1, and you have through and including Day 14 to use everything up.
You can skip days. You can spend a day doing NO PARKS at all. If you do this, NOTHING will be deducted from your ticket. Disney doesn’t care if you go on consecutive days, which is why a ticket is good for 14 days.
And please keep in mind that on a day when you only use WPF&M admissions, you are NOT using up a theme park day on that day. For example, you have a 10-day ticket with the WPF&M add-on. If on your very first day you go to Typhoon Lagoon (a water park) and no place else, your ticket at the end of the day has 10 theme park days and 9 WPF&M admissions remaining. I included this example to illustrate that using a water park admission doesn’t use up a “day” of your theme park ticket.