Mild dehydration can occur on long flights, especially when travelers cross several time zones; alcohol consumption can also contribute to dehydration. Generally speaking, dehydration is thought to predispose a diver to decompression illness because the washout of inert gas (nitrogen, in diving) is less effective in a dehydrated individual.
If there were a relationship between diving after flying and DCI, we would expect to see a great deal of decompression illness on the very first day of diving -- indeed, some data suggests that there are more accidents on the first day of a planned multiday dive trip. Of the 88 cases reviewed from the Caribbean for 1994, 33 -- or 37.5 percent -- occurred on the first day. The remainder occurred on days two through seven. Given that there are thousands of tourist divers who fly to Caribbean and Pacific dive sites, these numbers are far too small to establish a cause and effect.
Although no one can insist upon a 24-hour waiting period after flying, such a conservative approach to diving after flying is a reasonable idea -- it gives divers an opportunity to rehydrate, adjust to a new climate and time zone, and rest up after a long flight.