If you haven't got these already Windxtasy, here are the tide time tables for Peel Inlet taken at the Science Platform (page 91-92), which is close to the middle of the estuary. Add another hour for Fangys and Boombas. These are the most accurate, but because both spots are highly influenced by storm surge, barometric pressure and prevailing wind, use them as a rough guide only :-)
www.transport.wa.gov.au/mediaFiles/marine/MAC_P_TidesComb2014.pdf As a rudimentary guide, whatever the tide height is, that is roughly the depth of water over the outer bank at Fangy's.
Storm surge is historical (updated day after) but gives a good trend indicator:
www.transport.wa.gov.au/imarine/mandurah-storm-surge.asp