OK, so the key phrase in this discussion is "added it at checkout" that's throwing me...
Like you,
@nickys I tried to go into the WDW ticket sales website and go through the steps. I went in and started a purchase for a 4-day Park Hopper ticket. I got all the way to the purchase screen where you enter your credit card info and did not get an "upsale" offer to add a water park ticket to the purchase. That's not to say that the OP *didn't* get offered that, just that *I* didn't see anything like that as I went through the steps.
However, there is an option back during the ticket selection screen where you can upgrade to the Park Hopper Plus feature, which mentions access to water parks, that I suppose could be interpreted as "adding it at checkout".
Here's what that screen looks like:
View attachment 303625
This would then add the water parks for only $25 on top of the Park Hopper ticket price and make the ticket a "Park Hopper Plus" ticket.
So the question is whether
@ELG13 purchased a 4-day Park Hopper Plus ticket? Or a 4-day Park Hopper *and* a 1-day water park ticket. The easiest way to check is to go into your MDE account and look at the ticket(s) assigned to your profile. Is it a single ticket that says "Park Hopper Plus"? Or two separate tickets?
If it's two separate tickets, they are totally independent of each other. You can use them on separate days or the same days. (And if this is what you bought, you also got hosed on the purchase since a 1-day water park ticket is at least $60 which is more than $25 you would have spent upgrading to Park Hopper Plus.)
If you did indeed buy the Park Hopper Plus, then you have 4 days of park hopping in the four main theme parks. You also have a *separate* pool of 4 (yes, four!) admissions to the water parks. These admissions can be on the same days as theme park visits, or on totally separate days from your theme park days. They have no bearing on each other.
The only thing is that *all* admissions, both theme park and water park, will expire 14 days from the first day you use the ticket to enter any park.
Hope that helps!
-Rob