Smooth seas make the voyage more pleasant.
A ship in port is safe, and that's just what ports are for.
The advanced rules specify that supply must trace back strictly via railways or rivers- NOT roads.
However, Alan Emrich's "Living Rules" (and earlier errata) corrects this, and indicate that supply can be traced along road segments, providing there's friendly forces at each end of each road segment used in the supply line.
There's only a little discussion of this, hereabouts. I'm surprised- does Phalanx's supply rule actually work, as written?
Are most people are using the Phalanx rules, or Alan Emrich's updates?
No idea what most people are doing, but I use the Living Rules (plus updates Alan has given in ConSimWorld--e.g., revised Union setup, which may or may not yet be in the Living Rules).