They make some pretty big yachts.
Or they could freight it across in severl trips, or tow a little bit at a time behaind the boat under the water, making several trips.
Or they could ship it across the ocean, transferring it to a smaller vessel in sheltered waters in the dark of night.
The smaller vessel wouldn't even have to take it aboard: it could be fastened beneath the vessel: when submerged, the stuff would weigh less, and perhaps the water would help to shield the radiation from detection.
I seem to remember that the rubber raft that blew the hole in the Cole, they said had the explosives under, and not in, the raft.
There are a million ways they could smuggle WMDs, just as you pointed out. When people say 'can't', watch out. We should never forget how a cheap piece of wood made Japanese torpedoes able to sink ships at Pearl Harbor. FReegards....
Age, sorry but I don't give these guys as much credit as you do. They got a bunch of hits against a somnabulent US, which didn't even know it was at war. The US is engaged now, at least the executive branch is, and I think the terrs have a lot more than they can handle. I also think the technical aspects of a dirty bomb make it an impractical weapon.
OBL's biggest problem is that same as Tojo's was. He completely underestimates and misunderstands the US. It will be the cause of his early demise and the death of his organization.