Sydney harbour is "somewhat" larger than the Swan. And a lot of that area has mudbanks, it's not all deep like Sydney. traffic is restricted to the channels Looking at it from anywhere around the CBD/South Perth/Melville Water it seems huge, but that area is the only one really available for the room required for seaplane ops. the rest of it is relatively narrow and winding. Very few really viable choices. Also remember that Perth is one of the windiest capital cities in the world. Even less options in the strong seabreeze (although certainly useful for coming "unstuck" in the takeoff run)