Lauren has an excellent answer, a ship's helm is as low-level as it gets, even today, but I think users are much more sophisticated than they were when Netscape was on top of the browser list.
I think one reason why you might have a problem getting a firm answer is that "navigation" is still only a verb. It isn't enough to say one is "navigating", one has to be navigating something—be it an ocean, the home directory on a computer, or even something conceptual like tax law.
"Navigation" really needs to be applied to an object or concept in order for it to have meaning, and knowing that is what really informs the type of icon to be used.
UPDATE per OP:
The "nav" icon would be smaller, but the more relevant issue there is that an icon doesn't need to be localized like a string would need to be (i.e., What is the translation of "nav" into any of the Cyrillic, Asian, or Latin languages?). An icon would be much easier to handle in such an instance (though the icon could change based on the cultural standards of the user).
Even then, you are still talking about navigating the interface of a device, which is different from a lot of other tasks, and that narrows down your choices quite a bit, which I see as a good thing here. The context of "nav" would change to meet the needs of the device and the application the button is seen in. Even taking @DA01's comment about the use of an eye, that could change in a camera or image-editing app in very short order.
I don't think you are ever going to find The One True Icon to represent the concept of "navigation".