http://www.waterland.gr/mainen.html

This is what you are looking for

Just back from Halkidiki yesterday, have been three times now as love the place.
The waterpark is quite a distance from where you are staying, are you planning on hiring a car? There are local excursion agencies that will take you there, but I think most of the Tour Operators dont.
We always stay in Hanioti which is a bit further down the finger from you and dont really bother with it as it is too far away. If you cosider that your airport transfer is around two hours then this gives you an idea of the distance as the waterpark is closer to the airport than Polichrono.
If you are going with a Tour operator there is plenty of other trips to do.
If you are going with First choice, I can give you the current prices and details of these if you want as I brought the list home.
Personally we dont drive in Greece as they drive crazy, one scare in a taxi in Rhodes put me off for life.