They're dredges, possibly working on the Tokyo cross bay tunnel. The southern and northernmost dredges have large hopper-bottomeded barges tied alongside, to receive the spoils (mud, dirt, and rocks) they dig up. The barges are then towed to the dumping site, the bottom opens, and they dump their loads.
Three of the four barges also have white, square objects at one end, probably some sort of frame or guide within which they dredge. If you look closely at the southern dredge you can actually see the bucket almost dead center of the white square.
There is a fourth dredge, with two cranes and a tugboat-propelled spoils barge alonside, two miles due east of the dredge in your thumbnail image.
Also visible, about 3/4 of a mile east of the group of four dredges, you'll see two tugboats pushing what appear to be spoils barges, possibly from a couple of the dredges.
Bays, No Longer There, Offshore 
Three of the four barges also have white, square objects at one end, probably some sort of frame or guide within which they dredge. If you look closely at the southern dredge you can actually see the bucket almost dead center of the white square.
There is a fourth dredge, with two cranes and a tugboat-propelled spoils barge alonside, two miles due east of the dredge in your thumbnail image.
Also visible, about 3/4 of a mile east of the group of four dredges, you'll see two tugboats pushing what appear to be spoils barges, possibly from a couple of the dredges.