Ok time for the boring dude who is going to post the facts about the place and deflate everyone's balloons. The coastal plain of the Perth region is essentially a sand plain with a bit of limestone here and there. As a result the estuary at Mandurah is not deep.The eastern edge has two rivers and some run off drains feeding in. It has a sand/silt bottom. At its eastern edges its becomes very shallow and indeed in summer when a High pressure system is sitting in the Bight and the easterly wind is blowing, it will be totally exposed.The gradient of the estuary floor in this area is very low.
Come late winter there is a lot more fresh water, oxygenated, cool, and with a bit of fertiliser/nutrient load. The tides are usually on the high side because of wind and barometric pressure during winter. The result is that the silted shallow areas grow weed faster than Cheech and Chong could ever imagine. Most of it grows from the bottom, however you do see free floating clumps.
As time goes on the weed gets longer and the tides get lower and eventually the holes in the cheese align and you get the conditions Shanno and Strop scored.
With the onset of summer the high barometric pressures result in these area being laid bare to the sun and the weed is burnt off and dies. So the ideal time for weed affected water conditions is spring. Once the weed is gone you rely on the shallowness to take out the chop. Both scenarios require heavily raked fins.
Finally the ideal wind is one that is close to on-shore. The fetch across the estuary takes out some of the turbulence associated with land structures.
Here endeth the lesson.