There is a pattern of ocean currents which is largely stable over very long time periods.
Although the speed of these currents at depth is slow (only around 1m/s) it is predictable.
Today’s invention is a flotilla of linked, submarine-like containers designed to transport goods around the globe using only these deep currents as a power source.
These could be made fairly cheaply of reinforced concrete and capable of carrying a large volume of commodities such as fuel or foodstuffs. The speed of transport would be slow and might give rise to futures-like trading in the value of materials in transit for eg 60 days between continents. Huge sections of these journeys would be effectively naturally refrigerated.
It might be possible to reuse some of the world’s surplus military subs for this purpose, or at least to head each flotilla with one so that its navigation systems could be used to direct the goods to the correct destination(s).