According to the this answer, in the US buoys on the right are red when returning to a harbour, with green on the left. This is in contrast to most of the rest of the world.
What caused this split in the conventions?
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There used to be very many more buoyage systems in use around the world which resulted in confusion and danger. An agreement to unify the system was agreed at Geneva in 1936 by the League of Nations, but the collapse of the League and World War II scuppered that. In 1965 IALA (the International Association of Lighthouse Authorities) created a Technical Committee to work out a solution.
In 1976 IALA region A rules were accepted which combined the lateral marks (red can to port, green cone to starboard when proceeding upriver) and the cardinal marks (N, S, E and W) and this was accepted throughout most of Africa, Australia and Eurasia except where there was a predominantly US influence (Japan, Korea etc).
The US (principally) wished to preserve its existing investment in buoyage. Changing would be expensive and a major undertaking covering all the coasts and rivers. Retraining of both commercial and amateur sailors would be required resulting in costs and uncertainties during the changeover. Therefore IALA system B rules were incorporated into the international rules in 1980. Shapes are common to both systems, as are the cardinal marks, isolated danger, safe water, special and most recently new danger. However in IALA region B the colours for the lateral marks are reversed: green can to port, red cone to starboard when proceeding upriver.
A good summary is available at: IALA: Maritime Buoyage System (Wayback machine) and also in most books on navigation published outside of the US after 1980. From my reading, some US books only concentrate on US waters and fail to mention IALA A, and also some include US internal conventions without always being completely clear about their extent.
@Martin's existing answer is good, but I'd like to emphasize a point behind his point: Basically, the reason that standards consolidation has not gone further is because it is more trouble to change it than to live with it.
As has been noted, the world started out with few or no standards, each harbor doing its own thing. As trade increased, there was value seen in having a standard and, sporadically, areas coalesced to a shared standard -- but not necessarily to the same standard each time! (This is a good example of a symmetry-breaking phase transition.) In particular, countries tended to settle on a single standard for all their own harbors.
As the size of a region covered by a standard increased, the cost of changing to another standard increased while the value of changing decreased. (The larger a region, the more of its trade comes from elsewhere within the region and gains nothing from adopting a different standard.)
So, in general, smaller standardized regions tended to merge to larger (or more powerful) regions, often adhering to the standards of their larger trading partners, while larger standardized regions tended to stay as they were.
The rules for air travel came to a different result because of different conditions. Why didn't this balkanization happen with air travel? Why does nearly the whole world follow one very standardized set of flying rules? Three reasons: First, air came in much later and setting an international standard had become commonplace. Second, after WWII, just as international air travel was really taking off, the US briefly dominated aviation and most countries found it easier to adopt the US system right from the start.
Third (and most important), the tempo of decision-making in the air is roughly 100 times faster than on a ship. A ship's master (i.e. the person in charge of the vessel at that time) can afford the time to look up the rules for an approaching harbor and even to double-check the rulebook when they see a buoy. A pilot approaching an airport and landing is busy and has no time at all to remember arbitrary local rules -- it's hard enough dealing with local traffic patterns which must be different in each locality. (This is why ATC uses feet for elevation -- meters might be tidier, but experience tells us that changing over would, for a time, add enough confusion and extra workload that would without a doubt result in multiple collisions and crashes.)
For ships, the cost of multiple standards is low. For planes it is not.
The bottom line is economics: The world has now reached a point at sea where the cost of retaining several standards is lower than the value gained from switching to a single standard.