Siam Park is easily one of the best water parks in Europe. The quality is excellent, not like some of these run down parks you see in parts of Spain and Greece etc. There was four of us on our travel and we booked a private cabana, I would highly recommend this as you get free unlimited fast track on all the rides, your own rubber rings so you don't have to queue and wait for some. You also get unlimited food and drink which is really ideal as the food is very expensive usually. The cabana itself was lovely, own private sunbathing area, shower, TV, fridge, all very private and hidden away.
The slides are great and suitable for all ages and the wave pool/beach is lovely!
Definitely worth a visit even if you don't book a cabana!