Ask first about the cost to add the water park fun but don''t do it immediately unless you get (approx.) $28. as the answer.
From inside a theme park or after having exited, the cost would be about $28. (for a standard 5 day ticket with hopping) or the upgrade would be impossible (for a non-standard i.e. promotional ticket).
If you had not used the ticket yet, you can change your mind, go back to the window, and trade the ticket in anyway for a ticket with water park fun, paying the greater amount you were told when you asked.
As a point of information, a separate one day water park ticket costs about $64. You can take one spent water park ticket (not used more than 13 days ago) and apply it towards water park fun on a standard park ticket, but no change is returned.