I don't think most people appreciate how automated commercial vessels already are. The ship navigates autonomously from port to port. If you have a twenty man crew on a commercial vessel, they are split across two 12 hours shifts, you may only have one officer on the bridge during on watch, a few engineers below supervising the engines and performing preventative maintenance while underway, and able seamen will be doing the rounds checking nothing amiss such as loads moving etc. The crew are largely there to respond in the event of a crisis as commercial vessels are such an expensive asset.

I think it's better to think of the crew doing underway asset management than "sailing" the vessel.

Container ships witch ICE travel 20 knots/hr.

Sail ships average 6 knots/hr and wind is less consistent.

There are some companies working on Kites to reduce fuel consumption. https://www.airseas.com/seawing

If the automation is there, is there a reason why more container ships at least partially utilize wind for locomotion? Is it just that retrofitting a giant sail isn't feasible onto existing ships?

Flettner Rotor sails appear to be becoming a thing, they partner well with electric ships, similar to how some electric planes use multiple propellers and EV automobiles can do independent 4 wheel drive.

https://en.m.wikipedia.org/wiki/Rotor_ship

Actually the analog to the all wheel thing might be:

https://en.m.wikipedia.org/wiki/Azipod

I forgot to mention sailboats, so zero fossil fuels.