No, you cannot use the GAD voucher to visit a water park but you can buy a plain one day water park ticket for about $48.
Redeem the GAD voucher outside a theme park you will be entering immediately, then at guest relations inside (or come back out and you have several days to decide) you can upgrade to add more days.
Upgrading the GAD ticket to another ticket with days (as opposed to annual pass type tickets) is officially not allowed until the ticket has been activated (used).
If you already went to a water park, you can take the (one) spent water park ticket, your regular ticket (including a used GAD ticket), and about nine dollars and put water park fun on the regular ticket for more water park visits.
(Regular including GAD tickets used more than 13 days ago, and pre-2005 tickets used at all, cannot be involved in upgrades.)