Pictures of the tanker I saw a few days back showed a very large indentation on the outside of the ships hull. Explosions can do funny things things to structures, but from what I saw, the bomber would have had to have jumped overboard and detonated the bomb just as he hit the water.
Maybe he was pushed by an alert crewman? Sounds like it would make an interesting tv episode.
It could be that the suicide bomber attacked in a small boat, as with the USS Cole.