No! This is as much a conceptual mistake as "interceptor costs 10X as much as the missile it is designed to destroy, so antimissile defense is foolhardy" without taking into account the value of what the missile can destroy.

Maybe the boat will get through; maybe it won't. The ship might be intercepted before it reaches the target. Or security personnel with geiger counters can find it on the dock. And so on. From North Korea's perspective (or the USSR's during the Cold War, or Russia or China now), the uncertainty of the success of such an attack makes it very, very risky to actually deploy, except maybe in a situation where you're already losing the war (and in that scenario, the odds of a successful detection by the target are obviously that much higher).

A ballistic missile, by contrast, cannot be stopped except with great, great difficulty. That's why North Korea has built missiles for its nukes, and not a fleet of cargo ships and fishing boats.

Ships are not currently inspected at sea, and it's impractical to do so. The cargo is in giant stacks, and you need a dock to unload it for inspection.

Finding it at the dock is completely useless. You can set the nuke off while still at sea and still destroy half a city.

There is no mechanism for interception of such an attack at the moment.

You need a neutron detector, rather than a Geiger counter, for nuclear material detection by the way.

That, plus cargo ships and fishing boats aren't pointy enough. They'll put a smile on the faces of the enemy.

That might work as a first strike capability - although sailing a cargo ship from North Korea to Tokyo without garnering any interest from intelligence agencies might be more difficult than you give it credit for.

But the main thing you want nuke for is as a deterrent. Get in a fight with us and we press the button. It's hard to imagine the sneaky boat trick working when North Korea is under blockade by the entire US and Japanese navies. And even if they run the blockade they're going to have trouble getting close to Tokyo.

Past history suggests they'll get away with it extremely easily: https://en.wikipedia.org/wiki/North_Korean_abductions_of_Jap...

Maybe they use typical smuggling routes and have the whole thing buried, in a storage unit, long term parking or rented apartment?

An interesting "cheap, dirty & dangerous" mode of attack is loitering munitions from commodity FPV drones. Their capabilities have been demonstrated in current Russian invasion of Ukraine, and if any infiltrator can get in a few miles of the target, a "suicide drone" with a few pounds of explosives and shrapnel can be very difficult to defend against.