Arguably the best forecasts for inland sailing is WillyWeather

Select "Southern River" from the list of locations.

The closest location to Southern River for weather observations is Jandakot. These are shown on WillyWeather, and can also be viewed in detail at:  

if we are sailing on the South Perth foreshore, select Swan River-Mends Street jetty in WillyWeather. The closest wind observations are from Melville Water (Inner Dolphin):  

There are lots of other useful weather sites, just Google SeaBreeze, Meteye, Windy, BoM WA etc.