How warships disguised their identity

How often did ships disguise themselves as objects such as icebergs?

Rarely; disguising a ship as another vessel or nationality and breaking up its outline were much more common methods.

Disguising ships as icebergs or other objects was relatively unusual. Breaking up a ship's outline was more common. Ships also concealed their identities by resembling merchant vessels or warships of another nation. False national flags offered another familiar means of misleading an observer.