Delivery platform? WMD could be delivered via a small personal craft or even a Zodiac. Why would anyone hijack a slow-moving tanker?
To use as a trojan horse to deliver high-speed attack craft to an American harbor?
I am sitting on the 20th floor of the San Diego Marriott right now (missing one of my 7 San Diego days/year in New Hampshire).
Across from my window, I can see a large CVN moored at a pier.
Yesterday, I took the harbor cruise, and got up close and personal with 20+ warships, including 3 helicopter assault carriers tied up side-by-side.
All I can think about, after seeing all this, is, "How is AQ going to attack those ships"?
I'm sure the tankers, if they exist, are going to deliver attackers, not blow up themselves.
The Coast Guard has a very difficult job.
You've answered your own question.