FYI,sea water from those places u mention is not as salty as what we use for our hobby hence we still need top up salt. As for cleaniness,so long no dirt or rubbish that can be seen by our naked eye should be fine. Anything microscopic should be able to skim via skimmer.
And I was told to get sea water that has the same salinity or put it this way directly can be use for our reef tank wan will be somewhere at harbour front. But required hose or pipe to go deep down 20m to retrieve it. It required some kind of licence to do so.