I have an outstanding source of filtered sea water here on oahu, and wouldn't use anything else. The salinity is a touch high, I dilute with RO to 1.025/26. The calcium and alk levels are naturally high. If the source wasn't filtered though, I probably would not use it. We always take chances using water straight from the ocean, hard to tell what might be in it bucket to bucket. One tainted batch and you might crash the whole tank. Also makes it difficult to control any bacteria/fungus you are introducing.
Its always a risk, but knowing that, its worked so far.