My niece and nephew went to the waterpark last year. Although it wasn't the best one they have been to, they still enjoyed the day.
We are also going in three weeks. We fly out on Sun 29 July for two weeks and staying at the Planet (where we stayed last year). There are 35 in our party - for the wedding of my niece!
Hi we are off to Zante on 3rd August - Cava Doro - mixed reviews but looks like it has been refurbished for this year.
My b in law went to Zante last year and visited the waterpark. His advice was go after 2.00 (its cheaper) plus you will do everything in there in about 2 hours.