Why is it easier to ship goods by water than by land? It is much, much cheaper to ship goods over water than over land. (Specifically, I'm referring to the cost per unit mass of cargo per unit distance transported. I'm also assuming the large-quantity and long-distance regime, e.g. shipping thousands of pounds of goods over thousands of miles; I know that the economics are very different for smaller loads and shorter distances. Of course, if you're shipping goods between continents then you'll eventually need to ship either by water or by air because there aren't any land paths between your endpoints. But even when you're shipping between locations on the same landmass, my understanding is that it's often cheaper to go a longer distance over water than a shorter distance over land.)
Of course, some of this difference might be explained by "human" factors such as tax policies and legal regulations. But as I understand it, this fact has been true for much of human history (which is why historically, most of the biggest cities and population centers have been located near ports) and is true in pretty much every nation today. This suggests to me that there's a fundamental physical reason for this fact, i.e. some intrinsic physical difference between the energetics of shipping by water vs. by land. What is it?
Most of the explanations I've seen simply say that you can fit a lot more cargo onto a ship than onto a truck or train, so you can take advantage of economies of scale and reduced labor costs per unit of cargo. This is undoubtedly true, but it simply pushes the question back to why the optimal size for a transport ship is so much larger than for a transport truck or train. Moreover, trucks and trains tend to move faster than ships, so they experience more air resistance per cross-sectional unit area, but this just pushes the question back to why the optimal speed for land transport is so much higher than for water transport.
I'm not really sure how the resistance that engines need to overcome divides up between (a) axle rolling resistance (for trucks and trains), (b) tire deformation resistance (for trucks), (c) water resistance (for ships), (d) air resistance (for all three), and (e) internal engine resistance (for all three).
Off the top of my head, here are a few factors that might advantage land transport:

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*You don't need to overcome water resistance, which (I suspect) is much higher than air resistance.

*You can travel over land, which probably results in more direct paths overall between many human population centers, since we live on land.

*The base of your transport doesn't need to be watertight.

And here are a few factors that might advantage water transport:

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*You can travel closer to a direct geodesic, since there are few obstacles in the water.

*The water level is pretty flat, so you don't need to waste energy going up or down hills.

*The water changes shape to exert bouyancy over whatever shape your hull is, so you don't need to have a flat bottom like you do with land transport.

*You don't need to lay down roads or track ahead of time. Laying down tracks and roads has a high upfront cost, but gets amortized across many many different trips, so I suspect (although I'm not sure) that this fixed cost is dominated by the marginal costs of maintenance and fuel. Moreover, these costs are often paid for by governments, so the shipper only faces the marginal costs of transport over existing roads. (It's true that most existing roads are not nearly wide enough to accommodate hypothetical trucks/trains the size of large container ships - but again, that simply pushes the question back to why the respective transports for the two modes are such different sizes.)

*The fact that they don't need to stay on roads also means that ships are less likely to get stuck in traffic than trucks are (I'm not sure how big of an issue traffic is for trains).

Another big difference is that water has much higher viscosity than air, but I could see this netting out in either direction; it means that ships can "push off" against the water in a way that isn't possible against air, but it also means that the water resistance will be much higher than air resistance.
It isn't intuitively obvious to me which of these sets of advantages would win on net. I suspect that there's one effect (either one listed above or one that I'm not thinking of) that totally dominates all of the other effects listed here. But I'm not sure which one.
 A: There is a nonlinear dependence of drag on speed through the water for a ship's hull- which asymptotically approaches zero at zero speed. This means that you can use a single mule to (slowly!) pull a barge weighing many tons along a canal, which the same mule could not pull if the same load were on a wagon on land (because as The Photon points out, the rolling drag of a plain bearing depends on the size of the load even at near-zero speeds).
This means for transport, slower by water is a lot cheaper in terms of fuel burn than the same speed by land.
In addition, the cargo-carrying capacity of a ship's hull scales with its volume while the drag forces scale with its wetted area. Since volume increases faster than area as a ship is scaled up, water transport gets a lot cheaper as you make the ship bigger, and since the only size limit to a ship is its ability to fit through the canals in its path, ships can be made way bigger than any diesel truck, and water transport is hence far more economical for bulk cargo than land transport.
A: Perhaps not the physics answer you were looking for, but an important point nonetheless: water is, for the most part, the same everywhere. Historically, I could build a ship in (for example) Holland, and it could sail to, say, Indonesia and back without really needing any adaptations for the distant regions. Outside of natural disasters and maybe ice if you get too close to the poles, water is water no matter where you are.
Compare that to land transport. My vehicle will need to navigate the watery soil of Holland, then go up and over several mountain ranges, through both snowy and desert areas, then through several thousand miles of dense rainforest before finally reaching Southeast Asia. Both the vehicle and crew will need to be capable of negotiating a huge spectrum of extreme conditions.
That’s at least one of the historical reasons ships were preferred to massive land vehicles, particularly once globalization began.
