Something not mentioned here: if your ship/car is headed for the west coast, you can watch it go through the Panama Canal! Get the name of the ship from your CA, and even though you often can't track it across the Atlantic, you can see the estimated arrival in Panama. Check the sites again around that time and it should be picked up in the canal area, and you can track its progress through the locks.
Panama Canal Live Webcams. Below is the Equuleus Leader with my 328d wagon on board, going through the Miraflores locks out to the Pacific a few days ago.
Once the ship is through, you'll lose track of it again. Apparently not much monitoring available through Central America. Hopefully it will pick up again when it nears the US.