Siam Park in Tenerife is a brilliant water park. Lots of imaginative slides - not just the normal ones and also a big wave pool/pretend beach etc. It's really good. I don't think there are hotels on site but loads very close by and the water park has a bus you can use.
My mate went to one last year in Egypt, said kids loved it but the pool areas got at too busy during the day and there was nowhere to go to escape the crowds. Normally their daughters had independence on hols ( young - mid teens) around pool but she said it was like going back to toddler days!!! Eek!! They also had their room broken into - daytime!! I looked into the ones which had the free watermark next door to the hotel, then u can escape if you want and have quiet days too.