Those "mounds" may be the barges. The bottom contours are calculated from a grid of depth soundings and interpolated - thus showing a rounded mound rather than the sharp contours of a barge. I also found this description:

"The barges sit upright on the bottom. Each is about 200 feet long and 50 feet wide. The top of the barges (the cargo bed) is about 15 feet above the sea floor. They sit parallel to each other and about 50 feet apart. The shallower barge is in 75 feet of water and the deeper at 85 feet. Their sterns point to the north.
These barges lie a quarter-mile offshore north of Four Mile Rock in about 75 to 85 feet of water. They are on a line from the Kingdome and the Veterans' Hospital on Beacon Hill (the southern-most tall structure on the hill), which intersects a line to the blue water tower on Magnolia Hill. You will be in front of a brown house on shore with a white house to the left of it."

This description seems to fit Erndog's find.

http://www.nwdivenews.com/MC/index.asp?showcol=Y&ID=106

No Lat/Lon but a sighting (replace Seabird stadium for the Kingdome. I may check these out myself in the next couple of weeks.