OK I think I answered my own question. You can download the mobile Navionics app and get a 15 day free trial of the subscribed information. The sonar chart coverage of the area is extensive, though not universal. It correctly depicts the narrow channel into Foggy Bay for example, but not the one into Culpepper Lagoon, shown at 1 ft deep which it is not. Interesting in that area, both sides of the narrow entrance have contours, but not the entrance itself. The shelf at the ends of some of the fjords are depicted, but the mudbars from the creeks not quite correct, change too fast I suppose.
It does certainly look useful. Not that you would depend on it with your eyes closed, but it would give you a much better idea where to look.