How a terrorist can deliver a nuke to a US city at a bargain basement price

1. Build a crude torpedo that has a good hundred mile range and is capable of carrying a nuclear weapon. It would most likely resemble a nearly completely submerged boat than a torpedo. Outfit it with a cheap GPS unit for guidance, and a electric or diesel motor with a snorkel for propulsion.
2. Rig it up so that it will stear towards the coordinates of a large US coastal city of choice and detonate on arrival
3. At night, drive a ship (some cheap freighter or something) as far into US waters as possible before being spotted by a Coast Guard ship. I suppose it would help if you can find some way of getting clearance as a merchant ship.
4. Drop that puppy in and let it take its swim. If Cuban refugees can get ashore in a fairly visible boat, then a little underwater device, painted black for camouflauge wouldn't have too much trouble, either.
5. Hope that the Coast Guard guys don't detain you in your coastal city of choice, because it will be set aflame by a wave of heat and radiation,swamped by a tidal wave, torn apart by a shock wave, and then rained on by radioactive water.
6. Watch how little a $120 billion missile defense shield does to stop it

To me this seems like complete overkill:

1. If a terrorist did somehow acquire a nuclear weapon why would he take the chance of it not going off, or not going off where desired, by placing it in some kind of homemade, self-guided torpedo/speedboat?
2. Even if they did want to, for whatever reason, deliver it by boat, why not just drive the damn thing wherever you want it. These people have demonstrated their determination and willingness to die on a number of occasions. I'm sure there are thousands of people who would jump at the chance to set off a nuclear bomb in an American port, even if they had to go up with it.
3. Again, if you want to deliver it by boat, why would you take the chance of trying to illegally penetrate the border? Why not just stick it on one of the tens of thousands of cargo/container ships that dock in American ports on a daily basis.

I don't think delivering the device is the issue; getting one certainly is.