There is one at a Hotel in Hisaronu, my family enjoyed
http://www.granducel.com/hotel/aquapark.html

I think there is another at Calis Beach

You can get to both by dolmus (might have to change at Fethiye for Calis)or go on arrange trips.

We went to the one in Hisaronu also and we loved it! another family in the same hotel as us went to a different one and came home at lunch time as they didn't like it - not sure if it was the calis one but they said it took a while to get there so might have been. It took us less than 10 mins and we spent the whole day playing on the slides, fom what I can remember the slides close from about 12:30 - 14:30 but this gave us time to have something to eat and get out of the sun for a while! We never queued for a slide all day and we were there around 7th July last year!
I would say go for the one in Hisaronu as it is very clean and quite small (6 slides) so you know the kids are never far away!!

this hotel is in fact in ovacik with the water park