With your 10 day MYW tickets did you not still have to complete all your trips to the parks and disney quest/water parks etc in 10 days?
US tickets are valid for 14 days from first use.
a 10 day MYW ticket means that you can go in 1 park each day for 10 days over a period of 14 days.
To change parks during one same day, you need hopper. You can't use a "second day" on your tickets to enter a second park on the same day.
Or do you mean you get 10 days worth of entry to the parks plus 10 days worth of entries to the water parks to use on top?
if you have a 10 days MYW + waterparks you indeed get 10 days worth of entry at regular parks AND 10 entries at waterparks/DQ (note that I did not say days for those, I said "entries")
But your tickets will still be only valid for 14 days.
So a 10 days ticket + waterpark is 10 days to the regular parks and 10 times to waterparks, it doesn't mean that it will last you 20 days. It will only last you 14 days no matter how many days/entries are left on those tickets once you hit day 14 it's your last day no matter what (I don't see the point in talking about non expiration option at that point)
So let's say you have a 10 days MYW + waterpark, you can do days with regular park + water parks, days with only water park, days with only regular parks
each entry to a waterpark or disneyquest will use 1 of your 10 "lesser parks" entitlements. each 1st entry of the day to a regular park will use 1 of your 10 days on your ticket
your ticket expires when all 10 lesser parks entitlements and regular park days have been used, or after 14 days, whichever comes first.
Otherwise, as my mind is seeing it right now, you would still only have 10 days of entries regardless of whether or not you included the water parks on your MYW tickets?! Everything on the tickets would have to be used in those 10 days??!
as said above, 14 days.
It's 10 days over a period of 14 days from first use.
during those 14 days, you can also visit waterparks 10 times. (note that waterpark option is only worth it if you do more than 1 waterpark during your stay. If you're unsure you will do more, you can leave that option out, and still upgrade your tickets at WDW and add that option for the same price you would have paid if you had purchased it with your tickets.