Complex sentence: Witnesses reported seeing steel drums floating after the sinking, leading to speculation that they did not really contain heavy water, but an examination of records after the war showed that some barrels were only half full, and therefore would have floated.
Split into simpler sentences: Witnesses reported seeing steel drums floating after the sinking, leading to speculation that they did not really contain heavy water. But an examination of records after the war showed that some barrels were only half full, and therefore would have floated.