a reservoir for emergency fresh water would sure go a long way for peace of mind, should you ever have an extended period of poor harbor water quality to wait through. But then, would making a reservoir large enough to be useful for a system of this size introduce more problems, like how to keep that water "fresh" and not stagnant?
Another thing to think about is, since this system will be wired into the native ecosystem, I'd strongly suggest that you proceed super cautiously with housing non-native species in your tank. Mother Nature has ways of defeating pretty much every human-developed preventive barrier. I say this after reading Time magazine's top-10 invasive animal species of the globe article yesterday. Even unknown hitchhikers on live rock or sand, or even fish, pose a chance of escaping out into your harbor via your plumbing system. I don't know if there is a fool-proof way of ensuring that EVERYTHING in your display tank stays out of the harbor, but it definitely warrants extensive wargaming sessions. Cirolanid isopods, invasive mussels or flatworms, the list of potentially harmful organisms that could harm the harbor water is really big. Sorry to be such a doomsayer on a thread that is awe-inspiring to say the least, but in taking on a project like this, you truly are accepting a role in your local environment's well-being. I hope this all works out and we can all celebrate your success on your "tank" build! Good luck!