A guy that I've bought some corals from in cali uses ocean water and has had pretty good success with it. Apparently they pipe it in from somewhere offshore.
I would think the main things to check out would be what streams, creeks, rivers and intercoastal are near by. Then what direction the ocean current goes from there. You'd want to pull your water from as far as possible from any of those and/or runoff points.
Sounds cool. My dad used to have a tank that he ran totally off of ocean water ( a long time ago). All he kept was fish though and when anything died he could just replace it immediately for free. I don’t know how it would work out for corals...