Ticket usage is not tied to the days of your resort reservation. You can use your ticket for however many days you purchased (and corresponding number of water park or plus entries) anytime before it expires 14 days after first use. So as long as you're not checking out on that 14th day, you can use it after.
(I'm assuing you didn't buy the no-expiration option since that would have made your question irrelevant . . .)