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I drink enough every day when I swim in it (though admittedly after years of such abuse my gut might be a little more hardened than the potential client).
Shirley the boiling method would be enough to take care of most escherichia and friends? And salt as a means to preserve against bacteria for thousands of years blah blah.
That said @Tenderloin's suggestion of going a bit down the coast (or even out to sea) isn't a bad one. Might join a mackerel boat from the marina this week and get some from a mile out. I'd like it to be "Brighton salt" rather than "Eastbourne salt".
I presume there's no difference in salinity depending on depth of water?
Would you willingly drink sea water straight off the beach in Brighton?
Your salt would basically be that, minus the liquid.