The U.S. Coast and Geodetic Survey plumbed the commercial waterways of the U.S., keeping keels out of mud; this blanket is from a 1931 chart of the lower Hudson (see those soundings in the river?).
The Survey was absorbed by the National Oceanic and Atmospheric Administration, which today maintains a huge archive of civilian nautical charts.
100% cotton. Wash in cold water, gentle cycle, line dry.