Well, who's to say they'd use the LNG or oil for their purpose? I had family in Nova Scotia when the Mont Blanc, an Ammo ship, exploded in Halifax - killed by the resultant "tidal wave" that wiped out their village...
A lot of damage could be done with a ship - I wouldn't dismiss this casually, but then I wouldn't get paranoid either ;0)
Texas City was devastated in the early 1940's by the explosion of a ship in the harbor.
The greatest non-nuclear explosion I know of was the Port Chicago, CA disaster in 1944. Two ammunition ships and an ammunition train went up, creating a hellacious kB! that intrigued Manhattan Project scientists. The blast has been estimated at about 5 kt. Roughly 300 men lost their lives.
FMI:
http://www.portchicago.org/ http://www.cccoe.k12.ca.us/pc/welcome.htm My point: while an optimally placed blast can do a lot of damage, a surface-level blast is not going to produce the dreadful body counts that some fear-mongers are trying to carry over from a nuclear-war scenario. remember that nuclear explosives are much higher yield than this, can be delivered within one metre or less of their intended detonation point, and can be set to function at an optimal altitude above the surface.
None of these things can be done by terrorists with a ship. They can't get their blast up above surface level, they can't get it into optimal position, etc., and they can't get more of a bang out of a boatload of stuff than is contained in the chemical bounds of the material.
Plus... as the anecdote someone posted about Boston notes -- those who are sworn to be vigilant, are still so being.
d.o.l.
Criminal Number 18F