Time Out NY has an excellent piece titled It Happened Here, an informative romp through NYC history & geography. Greenpoint’s contribution includes this entry:

The ironclad Union warship the USS Monitor, which battled the Confederacy’s Merrimac to a draw in Chesapeake Bay, was built at the Continental Iron Works shipyard in Greenpoint and launched on January 30, 1862. The shipyard was located on Newtown Creek at Cayler and West Sts, Greenpoint, Brooklyn.

But it didn’t happen there (or rather there is no there there) – Continental was located on Bushwick Creek, not Newtown Creek.