Apparently we have been aware of that threat for a while.
Just found this earlier article:
Terrorists May Use Scuba Divers, Planes
http://abcnews.go.com/sections/us/DailyNews/homefront0020524.html W A S H I N G T O N, May 24, 2003 Terrorists may use scuba divers or small planes to attack targets in the United States, the FBI said today in the latest in a series of unconfirmed warnings.
"Recent information has determined that various terrorist elements have sought to develop an offensive scuba diver capability," the FBI said in an information bulletin issued by its National Infrastructure Protection Center.
We've gone to high alert a couple of times over threats coming from the sea.