the poo shoot is in the river opposite the terminal, current carries any water from there and the refinery out the river and north. I've spent a bit of time in and around that area and the water is very clean.
My point to the OP was that his trail to the boat ramp covers some very dry area, if done by boat it would be shorter to go to either the Wynnum creek boat ramp or go the other way around Fisherman's Is to the boat ramp marked on the top map. It is these large areas of banks that cause the calm water.
What beach is there at the end of the rock wall is another question