You cannot navigate without internet because the map is downloaded as and when needed. Like Bro Alyem said, the Live informations is excellence. I guess precache means one have to exactly follow its route or else you with have a waze opening page only without the route map.
This to answer hmaxx too.
To precache a map if you do not have 3G is, get in to a wifi zone (maybe your home) and do a "Navigate to" key in the keyword let say "London" it will show london and just maximise the screen and wait it will precache London and surrounding area, just scroll around to precache the entire British Isle if you want.
Once you are in London, you just open up Waze in airplane mode, with only GPS on and use it. No live traffic but you get Nav.
Obviously you don't need to go to London and try, have a go locally.. Precache as mention, set your phone not to receive Data or in Airplane mode and try to Navigate.
Example of Precache over Wifi Navigation in airplane mode done in Tarakan, Indonesia.