The boat ramp at South Durras happens to be in the corner of a surf beach, so its not like the average boat ramp and it gets cleaned out tidally. But, yes I do generally avoid collecting from boat ramps.
But, equally as bad is collecting from spots either located in bays/river systems and/or located near the entrance to bays/river systems.
Then again, if you consistently collect when water is more likely to be clean and your corals get use to the water you use (i.e. fussy ones die and you keep the ones that are happy in your system) then your tank will be acclimatised to whatever you use.
I have written previously about collecting water at Malabar, a spot in the Eastern Suburbs of Sydney. The north boat ramp is a very convenient spot to collect water and I have seen a number of commercial collectors collecting from that spot. I do not collect from there anymore and pity those who pay for the water that comes from there. I have seen water being collected on days I wouldn't even swim in it. To be fair, the collectors wouldn't know the condition of the water unless they actually dived in and looked at it. I would rather collect from the north side, near the rock pool, at high tide. The reason is the area from about the south boat ramp and further into the bay, although open to the ocean, tend to collect all sorts of crap. This crap is often mucuos-stuff that is probably natural, but might be from jellyfish etc. There is also runoff from the golf courses (let's not go into the chemicals that are used on golf courses) and apparently some sewage.
The north side of Malabar is far more exposed and the water comes through much cleaner.
My biggest problem with collecting from South Durras is sea weed.
Does anyone filter the water they collect?
I wonder if one of those sump filter socks could be used?