The transit time for a container ship across the Pacific is less than ten days.
And such an attack would only work once. All ships would be halted miles off of our coasts after the first successful blast.
The Chinese have a missile laucncher that fits into a container. No such luck. They could deliver a good many missiles that way and our SDI system would be worthless.
As for a bio attack...bio agents are piss poor military weapons.
But very effective for causing domestic panic, as I said. So far, all you've done is reinforce my point.
You can't put nukes onto enough container ships to gain global domination.
The risks are huge. You'd have strategic assets in transit for DAYS that would subject you to immediate obliteration if caught - something that a shipping fire or sinking could easily do to you, and accidents happen on the high seas.
And even then, you'd only be taking out a few coastal cities at most before being obliterated.
And that's presuming that your atomic trigger isotopes hadn't decayed too much, that your electrical circuits and conventional explosives on your nukes had survived the radiation during the transit - sans clean room maintenance, that the slightest bit of moisture hadn't rendered the heavy metals of your atomic core and shell into useless rust, and that your core and shell hadn't been cracked by getting bumped in any rough seas (plutonium and uranium are among the most brittle metals known).