Does a replacement propane supply line have to be made of a special material?
Simple answer - yes, it needs to be rated to supply gas.

Two reasons:


water molecules are bigger, so permeability is less of an issue for water, hence your hose pipe may actually allow gas to escape
gas is explosive, so any mistake here may be considerably more serious than getting a water pipe wrong


If you need guidance, call your local gas company for advice - the good thing is that these pipes aren't expensive, so you'll pick one up for very little.