In this world of all things electronic as regards ships navigation the fall back is the basic Mk1 magnetic compass or the compass binnacle. As most things at sea are subject to massive corrosion from the salt water then materials such as brass are well used to prevent thism as well as prevent undue exterior magnetic influence.
On most large ships, the helmsman stands on the bridge and steers via electronic means but has a magnetic compass as a standby and this is mounted on the exterior of the roof of the bridge with the lower portion projecting downwards, through the roof (deckhead) into the bridge and into the helmsmans sightline.
As most large ships now are made of steel they are subject to their own magnetic field and thus influence a magnetic compass. This is where something called Kelvins Balls are used to alleviate magnetic influences on a magnetic compass.
These are generally iron spheres and painted to prevent coorrosion either side of the brass binnacle.
Example thus -
http://www.acecrc.sipex.aq/access/page/?page=afcfbb2e-b886-102a-8ea7-0019b9ea7c60and :-
http://en.wikipedia.org/wiki/BinnacleThat gives the monkey island and brass balls colloquialisms as in use today.
Nowt to do with cannon balls per se, although there is a similarity.
