For scuba diving, Rottnest Island is probably your best bet (it’s a quick ferry ride from Perth or Fremantle). Through traveling north (towards Monkey Mia) would be good too. July is also prime time for whale sharks in WA, which is definitely worth looking into.

If you can, I’d recommend traveling south also – the Margaret River Valley has a lot to offer (beaches, caves, chocolate, and wine…), and the southern coast (Denmark/Albany) area is gorgeous – and home to the Valley of the Giants Tree Top Walk. Ocean Beach (near Denmark, I think) is on the Southern Ocean, and is absolutely beautiful.