There are several ferries to Rottnest and this is only one of them. Visit a tourist information office or buy a local newspaper and read the ads in it to find out from where and when the ferries is leaving and how much it costs. From Freemantle, a little bit south of Perth, is it only taking about 30 minutes to reach Rottnest. This ferry is leaving not far from the train station so if you are in Perth, you can take the train to Freemantle and catch this ferry. Then you will get another fun thing to do, go by train and see Perth's other suburbs.
Pictures from the area, click to enlarge the pictures.