It's my understanding, from some Disney podcasts, that while regular tickets expire after 14 days, Waterpark tickets do not expire and can be used anytime during a future visit.
This is from Mapleleaf tickets:
Q. When can I use my Water Park Fun & More admissions?
A. Your Water Park Fun & More admissions may be used anytime BEFORE, DURING, or AFTER your theme park days are up. For example: You may start the day at Epcot, and after lunch go to Typhoon Lagoon. You do not need a hopper ticket if you're only visiting 1 theme park that day. Keep in mind, you may use more than 1 Water Park Fun & More admission each day. For example: You may start the day at Typhoon Lagoon, and visit both Pleasure Island and Disney Quest at night. In this case, you have used 3 Water Park Fun & More admissions in one day. You also can repeat an option park. In other words you are not limited to only visiting an option park one time.