Individual tickets are expensive, so it's better to buy a combined ticket. Except for the Sydney Tower, the other three attractions are all together. The aquarium doesn't have a huge variety of fish, but there's a large shark enclosure with other fish besides sharks. The nurse sharks look very fierce, but are said to be very docile.
Finally, the penguin exhibit's boat ride is a complete rip-off if there are long queues. I queued for half an hour, only to have the experience last a minute. I was in 0°C heat, just pulled out my phone to take a picture, and in the blink of an eye, it was over. Then I had to queue again to get off. It was truly a nightmare.