Some people do it with no problems but you never know what pollutants you could be adding.  Unless you can test for everything there's always a chance of something nasty getting in. Did you ask what his filtration was like?..., I'd do it if I lived in Hawaii or on the great barrier reef 

Hi Chris.  You'd possibly also have to check the salinity too over a few weeks at different times to ensure that it is all sea water being at the mouth of the rivers , salt and freshwater can sit in layers in the river mouths and estuaries .

He did say it was important to try and collect your water from the incoming tide and advised me to go up to Fortrose to collect as the water up there is perfect!

Looking into it there seems to be a few people who have done this and is really like to give it a go. I have a spare 64l tank that I'm thinking of setting up and filling with real saltwater as a wee experiment before I go any further though.

Certainly worth a shot , yeah out passed Fortrose would be best bet , if your going to try I'd probably use a pair of waders so that you could get out passed the wash would probably be less silty .
I think the dilution thing comes from difference between 1.025 dSG we measure our salt at , and 35 ppt of natural sea water not being the same salinity .