I wound up going to Assateague/Ocean City for other reasons. I checked a few places and the only thing I found that looked suitable for snorkeling was bayside in the national seashore. Visibility was about 15 feet, but I was surprised at how shallow it is – I couldn't find water deeper than 3 feet even going out about 100 yards! And I was told it's about the same at peak tide. Did not see much – most of the vegetation was dead. But did run across some colorful orange sponges, a horseshoe crab, and two fish. (One of which appeared to be a "mono" (family Monodactylidae); these don't occur naturally in the Western Hemisphere, so I presume some aquarist dumped one there – it came over to briefly check me out, as if it associated humans with food.)
In any event, I may check the vicinity of Kent Island at some point – probably next year. I do recall in the past seeing some water with good visibility at Terrapin Nature Park and CBEC.