A lot of places give discounts to local residents and I mean in all countries not just Cyprus. Tourist places such as Egypt gives 3 different price ranges for foreign visitors, students and also local residents.
Some places in UK give discounts because of inconvenience due to lots of visitors - ie sports facilities etc. Given tht the waterparks are probably full of holidaymakers only in the height of the season - July & August, it's the locals who can keep them occupied in the mid-season months - May, June, September & October.
Fasouri waterpark in Limassol used to offer vouchers through Macdonalds for midweek visits as that's when they are usually quieter. Not sure if this is still the practice.
WaterWorld usually offer discount vouchers in the local English language newspapers, which you can just cut or tear out when you are here (Cyprus Mail, Sunday Mail and Cyprus Weekly).
The prices last year were around Cy£17pp for adults (approx 29 Euros), if that helps. There is a reduced price for kids up to 12yrs if I recall correctly.
You could see if the prices are on their website too