Oh. You mean about the same size as a small load of pot or cocaine brought in from the Bahamas, Mexico, or Cuba?
Yes, but interdictions are so frequent that it is not recommended. It has been suggested that the Chinese would need to smuggle two such devices into the ports of Bremerton and San Diego, to neutralize them prior to any actions against Taiwan. But it would be difficult even for them.
The bottom line is that a terrorist nuclear attack against the US would not be the best application of such a weapon.
If the weapon was Shiite, its primary target would either be a US carrier fleet in the region, Port Saud Egypt or the US naval base at Naples, the Saudi oilfields, or Tel Aviv. Any of those would be delivered by missile or fire ship.
Any ship carrying such a weapon for Sunni terrorists would be stopped at the US Coast Guard exclusion area, between 200 and 300 miles offshore.
And this assumes that they could maintain OPSEC during the project. If they didn’t and we or our allies learned of it, we would not be in the mood to take prisoners.