Hiya jane,
there are 3 water parks in marmaris, theres star aqua park which is a dated water park small and quiet, theres one near the marina which ive never been to, but the one i would suggest is auqa dream, its only 2 years old with all the facilities you need, its on the hill behind Mcd's/Migros the dolmus from
icmeler goes right past mcd's/migros. then you walk up the hill. when you get in they give you what looks like a credit card, you put money on this card, which you give to the bar/cafe/waiters and they deduct the charge. if theres any credit on the card when you leave, they cash that in for you, Its a great water park where you can spend all day, just beware, as its on top of a hill, its a huge sun trap and theres a breeze so you dont feel the effects till later.
hope this helps
wayne