That's Stormriders, and the part they are running ( or crawling depends on the scale ) around in what is described in Quelbourne.
While being allegedly the best book of the serie for the Kulthean feel, it's not really what can be called cannon.
What I do usually is the following :
- ground travel in civilized areas don't really need a Navigator, but if you're willing to pay for one you can, as it will provide some protection/warning from elements like flowstorms, rain, thunderstorm, hail, sleet, and so on... and it will also allow you to jump somewhere safe, if you're willing to pay for the jump, at will.
- ground travel in uncivilized areas don't normally require a Navigator, but having one at hand is just great for the above reasons.
- sea travel in closed seas or in a bay or near the coast don't really need a Navigator either, in case of problem, just run for the shore and wait for the problem to pass.
- You can try high sea travel without navigators at your own risk, it is not recommended.
While Flows can become impossible to pass through, it's not their quiescent state, so they are impassable for a small bit of time ( and things can be interesting during that time ) usually during a Flow Storm... That's my general rules, such things don't apply to several places ( the Pillar of the Gods, Nexus, Karilon, Ormian, The Vaults of the Eyes... that will each require it's own mean to reach it. )