Usually I go the for shopping only. So as for the waterworld im not sure. I only know it's open 10 am to 7 pm Tuesday to Sunday. Monday is closed and Friday they open from 3 pm to 7pm.
Megamall stuffs are slightly more expensive, but worth the buy for it's food. Recommended to buy the cakes from Breadtalk, nice and thou it's about same price as sg, but far bigger. And also J'Co donut. For clothings, Number 61 is like their giordano. Cheap and not too bad. There's a departmental store there, Matahari, but prices same as sg, not worth going in.
You can take a taxi to Nagoya Hill Shopping Mall. Cheap toys, clothes, shoes, etc. Their target customers is singaporeans. So be careful eh! The supermarket has lots of cheap grabs.
DC Mall is quite small, but cheap if you're gonna get cosmetic stuffs. and also a shop selling cheap clothings. bought a cotton singlet and pants for my girl for only about sgd$0.30. They also have their local snack.
Taxi is less than S$10 per trip. Better is dont try the die die negotiate method with these taxi drivers. Enjoy your trip!