Forget IW support, you've come to the right place. I was having the same problem, basically.
I believe what you have is a corrupt .ff (fastfile) file from what you are experiencing.
Are you loading it from the CD or did you download a no cd fix? Personally, I downloaded a no CD fix, because I legally own the game and hate having to find the CD everytime I want to play the SP. Not to mention my computer was having trouble reading the CD in the first place.
What I did was I browsed into the COD4 CD in my CD drive. From the "My Computer" directory, right click your CD drive and click "Explore", then when you're in the main COD4 Directory, there should be a Folder called "Zone" and inside that will be a folder called "English" that contains all the .ff map files.
The 2nd level should be "cargoship.ff". Copy this file from the CD and Paste it into the exact same directory on your Hard Drive. (Mine is C:\Program Files\Activision\Call of Duty 4 - Modern Warfare\zone\english)
This should overwrite the corrupt file and allow you to load the level.
I hope this will help you, but if for whatever reason it doesn't, please post here or PM me and I will try to help you figure it out.
Unfortunately right now I've been awake for about 30 hours, so I think I'm gonna get some sleep. I'll check back here in the morning.