The container ships might be efficient but something doesn't jibe with what you're saying about how high oil would have to go to make a difference.

Here's how I derived my estimate:

Bigger is more efficient and high oil price will drive ships as big as can be accomodated. I base my estimate on the numbers from Emma Maersk(and by extension its sister ships; over 10 of which are either completed or in construction).

It can carry 11 000 20-foot containers, 14 MT each. An empty 20-foot container weighs 2.2 MT. 11 000*(14 - 2.2) tonnes = 130 000 MT of goods.

It has an 80 MW wärtsilä engine as main propulsion with an efficiency of 52% and 5x6 MW caterpillar 8M32 engines(couldn't find the efficiency numbers so I used 38% fuel efficiency, which I estimated from a smaller 5 MW marine propulsion engine on caterpillars website). Maximum fuel consumption is 80 MW/0.52 + 30 MW/0.38 = 233 MW.

It has a top speed of 29.3 mph and a cruising speed of 21 mph. To sustain the top speed it will presumably use all engine cranked up to maximum, consuming the full 233 MW.

29.3 mph is 13.1 m/s. Fuel intensity per km is 233 GW/(13.1*10^-3 km/s) = 17.8 GJ/km.

Dividing through by mass we get fuel intensity per weight: (17.8 GJ/km)/(130 000*10^3 kg) = 135 J/(km*kg). To get a feel for how efficient that is; moving a 1 litre carton of juice 1 km costs as much energy as your body consumes in ~1.5 seconds while you asleep.

Right, but you're not going to ship things at maximum speed because drag goes as the cube of speed and as a result mileage will be proportional to the square of speed(ignoring slight variation in energy efficiency for the time being.). At 21 mph, which is the cruising speed of Emma Maersk, fuel intensity will instead be ~135 J/(km*kg)*(21/29.3)^2 = 69 J/(kg*km). Average engine efficiency including the caterpillars is 48%, with caterpillar auxilliaries turned off it is 52%; adjusting for this we get: ~69 J/(km*kg)*0.48/0.52 = 64 J/(km*kg).

As you mention below, slowing down is a common way to mitigate fuel costs. Slowing down a little bit to ~18-19 mph would bump that number down to 50 J/(km*kg).

The top of that fuel intensity range will be avoided when oil is expensive, so I took 50-100 J(km*kg) as a reasonable estime of fuel intensity for current technology.

A t-shirt weighs about 200 grams and half the earth's circumference is 20 000 km. Using the above fuel intensity that comes out to 0.2-0.4 MJ thermal energy, which is 6-11 ml of oil, which comes to 1-2 tea spoons.