I've pretty much decided to make the call to Garmin tomorrow. It's going back for a refund. Force4 will be given the whole story, and the names of the useless jobsworths and arrogant oafs at Garmin.
Spoken to Garmin and Force4 so that I won't have any quibbling when I take it back.
I did, however, find someone at Garmin who actually cares, but unfortunately can't fix the problem before my 276cx goes out of warranty. Speak to Robbe. He does know his way around the units, but between us found that we'd tried everything and still can't get it good enough to race with.
The chart problem is noted, but of course won't be fixed because you can just spring for the updates. I'm out, as they say.
My Garmin chart as per above. I would agree that whilst the barrier is marked, it's not particularly obvious and bizarrely the undersea cables are far more obvious.
I suspect you're already aware, but known 'issues' with the Navionics on iPad as follows.
Doesn't display AIS
You'll likely need some kind of waterproof case if you're looking to use it in the cockpit