There is no way to do a water park Annual Pass renewal online. If you show up at a water park ticket or Guest Relations window within 30 days on either side of the expiration date you can get an AP at the renewal rate and the expiration date will not change. If more than 30 days after you will be charged the price for a new WPAP but the expiration date will change to a year from date of purchase.
However, it is not to say that if you come a few days late, and checking the pass shows it was not recently used, that the CM in the window may sprinkle some Pixie Dust. We are allowed to use our judgment and sometimes bend rules. Just don't quote me on this paragraph.