The last time i got a taxi from Lindos to Faliraki was 30 euros so i'd expect a fare between 35-40 euros from Pefkos.
You could also get the bus which will be considerably cheaper. Not all buses go by the Waterpark (the majority just go along the main coastal road) but even if you get off on the main road you could walk to the taxi rank or flag down a taxi for the waterpark for around 5 euros.

I agree with the taxi prices of the person who posted before me. I stayed in Pefkos and found a nice cheap and friendly excursions agency up the road who sold trips to the waterpark for a third of the price. I can't remember what it was called or where it was, but I will ask my partner and see if he remembers!

Alternatively, you could take the free bus to and from Rhodes town to the water park and then just get a taxi to and from Rhodes town. May work out cheaper? If you google 'rhodes water park' and go on the waterpark website, it will give you the bus times, etc.

Hope that helps!

brill thanks for that info...we are going to get to pefkos first and then price all the options up...