Sorry guys and gals, but not blue crab larvae - wrong time of year, wrong place, wrong everything. The more likely scenario is they are grass shrimp larvae, which are quite prevalent in the middle and lower bay, and frequently inhabit the shallows and reside among the marina pilings.
Blue crabs spawn near the mouth of Chesapeake Bay, mainly in a vast area near Cape Charles, VA, a location where until recent years the sponged females were targeted by commercial crab draggers and sold to the picking market. The eggs were merely ripped off the crab and tossed in the trash at the picking houses, while the meat was Pasteurized and canned. Because the crabs were dragged from beneath the bottom in areas consisting of sandy mud, the meat was frequently contaminated with grit from the sand.
Crabs grow very rapidly after hatching. Much faster than anyone can imagine. The females only live about a year and a few months, while the males often live two years. The females migrate down the bay beginning in early September, and for years many of the Crisfield commercial fleets targeted them for local picking houses. The harvest was beyond a doubt one of the things that led to the species incredible decline, but legislation to protect blue crabs, like most natural resource legislation, became a political football, prompting a huge number of meaningless, action-delaying studies that lasted more than two decades. Consequently, the stocks nearly collapsed before minor harvest restrictions were put in place, and as usual, the restrictions mainly targeted what is referred to as "Chicken Neckers", recreation crabbers using collapsible crab traps and handlines baited with chicken necks. DNR made the ridiculous claim that recreational crabbers harvest as many, if not more crabs, than the commercial crabbers.
Back to the critters. There are lots of neat things swimming, squirming and crawling around on the Chesapeake's hard and semi-hard substrate, some of which you can readily see with the naked eye, but there's lots of more that you can only visualize with the aid of a large magnifying glass or microscope. The vast majority of these critters are various forms of zoo-planktons, which will really get your undivided attention.