Yes, the ships were definitively the fastest way to travel back then. Still, they were rarely sailing out of sight of the coast. You had no compass nor reliable maps, so getting lost was very easy. The Sun, the stars and a good eyesight were your main guidance, so you had to be decent in astronomy. Most ships were anchoring during the night. You had no way to detect floating debris like logs (common near the coasts), the oarsmen and the crew had to rest. Also, night was related to the forces of darkness, and I guess that sailors were pretty afraid of it. I've been sailing at night with a modern, safe sailboat (but without radar) and I can tell you it is at first pretty scary.
Carts loaded with goods were very slow. On roman highways it was ok. But on other areas, you had only muddy paths, and after a good rain carts were problably sinking in mud. They were much slower than a legion, which was trained to march quickly all the day long, or horsemen courriers, probably the quickest travelers of those times. There were "highway stations" along the roads to get a fresh horse, rest and have a quick meal, something even have a bath to clean the dust from the road, those stations were called "Mansio".
And yes, the amphora were definitively re-used. I'm currently writing an article on the amphora craftmanship in southern Gaul. Wineyards (villae) had often an intense pottery activity, in fact they produced much more than their needs. Which means that the container itself had a value and was sold empty, and I'm working in finding clues in various archeology articles to demonstrate that.
---------- Post added at 05:53 PM ---------- Previous post was at 03:37 PM ----------
I want to add that Gauls mastered the wooden barrel "technology" and were using it widely. So as said Urwumpe, the amphora was really a kind of "international" container. What is very different of today is that they were almost an art form, with each region producing its characteristic shape. The amphora I'm mainly studying had a capacity of 30 liters, and an empty mass of 10 kilograms. Even in Gaul, there is easily 15 known shapes of amphora. Today's containers look perfectly identical, regardless of where they are manufactured.
You also had the dolium, a much more massive container, which capacity varied from 1200 to 2500 liters. Of course, given the technology available, those were used almost exclusively in warehouses, where they were typically half-buried (to keep the temperature constant). But some have been found empty in shipwrecks.
About boats, now I'm thinking to it, it was almost suicidal to navigate at night. You had no lighthouses (except in very large harbours, like the famous one in Alexandria, Egypt), no beacons, nothing. And they were navigating near the coast, were all the dangers are (not only floating objects, but also and mostly rocks !). Only a perfect moonshine could make this possible. I guess they already had the idea of probing the seafloor with a rope and a lead weight, but that technique usually warns you when it is too late !
