Sea freight is the least polluting one - due to the extreme amount of cargo. Air is expensive and very pollutant.

However, talking about chips that are hundreds of watts each the pollution produced by them is a lot higher than any transport.

But the boat is already going back empty

There are moves being made to test ships with modern "sails". Here's a paper published about a cargo ship fitted with 4 sails in 2010. The findings are interesting with it achieving up to 25% better fuel efficiency when using the sails.

https://www.stg-online.org/onTEAM/shipefficiency/programm/06...

No, much less pollution. It costs less carbon emissions to ship from Shanghai to California and back then for one person to drive 10 minutes to the store in their personal car.

It's many orders of magnitude more energy to fab a chip than to ship it across an ocean.

TSMC alone accounts for 12% of Taiwan's electricity demand, and growing fast:

https://spectrum.ieee.org/taiwan-semiconductor ("TSMC’s Energy Demand Drives Taiwan’s Geopolitical Future" (2024))

From the US to Taiwan? I mean if Elon Musk want's to build a hyperloop through a sea floor spreading zone, then sure why not.