Without a question, one of the best beaches in Perth and my personal favorite for snorkeling is the Trigg Beach.
This beach caters to surfers and body boarders, but a small majority of swimmers hide away and swim north of the rocky point knows as the Trigg Island. These low wave rocky islets are ideal for a bit of snorkeling and splashing around. And once you work up the appetite, take a walk up to Yelo for a bite to eat. Then return to the beach or laze around the park next to it.
Trigg beach is easy to access by bus or car, with plenty of free parking available.