watch cap

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A dark blue knitted cap worn in cold weather, especially by enlisted naval personnel.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A knitted cap, such as the one worn by a seaman when on watch.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a knitted dark blue wool cap worn by seamen in cold or stormy weather