Disney doesn't count the water park admissions as being worth anything so you are better to hang onto that ticket and use it for water park trips and a single day at Disney at some point in time.
As I understand it, you have to use up your water parks before you use your last park day. If someone knows different, please let me know. I just heard this last month when we were there. I sent all to the water park to use as many as possible before we use up the park days.