Rookie members of the Nuclear Club could build a deadly low-yield device but it takes a ton of knowledge in metallurgy, electronics, miniaturization, etc., to build a (physically) small yet reliable device. Nukes are NOT easy to detonate because there is an awful that can go wrong.
The 'physically small' aspect is critical for delivery via missile, but almost irrelevant when you're looking at delivery via ship/submarine. Place a primitive nuclear device in the bottom of a ship's cargo hold, bury it in something that will absorb any telltale radiation (and perhaps enhance lethal fallout), and you have a workable first-strike weapon, just waiting for "slow boat" delivery to the target. The ship's crew wouldn't even have to know it's there. I imagine the US has been quietly watching for such threats for decades...