I know this area well. The sand is technically aragonite based, yes. You could certainly use it - clear it of the small coquina clams first or you'll have one heck of an ammonia problem. (Can you tell I've had the experience of rotting clams in my tank?)
I have not had trouble using substrate from the less-traveled areas of Playalinda, just dont get close to the dunes and stay well clear of the turtle nests that will start appearing about a month from now.
The water is a slightly different story, but I think its useable. I would not make a trip down to HBOI myself (a good two hour trip) just for saltwater, personally. You can collect from the shoreline, seive it to remove large plankton to prevent problems later on. If you want the plankton, thats a gamble. Its up to the person how far they want to take using a natural source.
I'm sure you know, but ocean water off a natural refuge and closed area doesnt guarantee it will be super clean. Its downstream of something afterall.

With the strong easterly winds we've been having, and the high surf, any coastal related pollution may be minimized. It might be a great time to collect.
I would not advise using water from the lagoon system.
>Sarah