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"I’d argue it should be zero."

So would I, but that's impossible in meat space for any industry or technology spawned in the last 200 years. I wouldn't mind technologically winding the clock back 200 years, using sail boats and letting my computer wither away (and livelihood, btw). Would you?

"This industry is one of the dirtiest, and least responsible to the world."

True. It's also, by weight and value of goods transported, one of the cleanest. It's a massive industry

Ships have the advantage of being able to dump waste in the middle of the ocean where there is little to no life (too deep, no light, no plants. There is some life). Compared to what other, land based, industries do this is comparatively benign. Here the waste gets diluted by an essentially infinite sink.

Infinite sink you scoff? Yes, depending on where and what you dump.

If you dump brine water off the coast near the Mississippi river, you're doing a lot of damage by significantly changing the salinity locally. Slowly dump the same brine in the middle of the ocean, and nothing will happen.

Similar for heavy metals. These dissolve into salt water, and were already present in solution since the dawn of time. We won't change those numbers significantly. (again, assuming you're not dumping it just off of land like the Japanese did in the 50s)

Plastic is a different ball game. Plastic doesn't dissolve and is carried, and concentrated by ocean currents. That's nasty.

Chemicals and isotopes that don't naturally exist are also nasty. Particularly if they have long half-lives.

High sulfur diesel is super nasty, and the shipping industry has to clean up, but it's not obvious how.

I mean, let's say we forbid them from using high S diesel. What happens to this diesel then? Do we dump it in tailing ponds? pump it back into the ground? It's not obvious what to do with this. At least at sea S emissions are spread out minimizing impact (as opposed to localized)

I'm not saying not to do anything. I'm asking, what are we to do? (I have some ideas in mind, so it's not impossible)

No, shipping is the most efficient.

Not only are drag loses comparatively small, those two stroke diesels are crazy efficient from a thermodynamic point of view (massive, slow, and high compression ratio -> crazy efficiency).