Whose recommendation? Not DAN's or any other recognized dive organization I know of. DAN has had the 18 hours after multiple days of diving for 10 years that I can remember.
Many dive computers have 24 hour countdowns but I suspect that's just to sell computers to divers who have heard some claim that 24 hours was safer - even tho there is no support for those claims as DAN studies found no real increase in safety by padding the extra 6 hours.
It's more a case of 180° opposite the case being true.
This is where a sense of history comes in. The
technical side of the "industry" has warped an existing and long-held theory to conform to the needs of the
travel side of the industry. Resorts were having a hard time telling (enforcing?) guests to stay out of the water -24hr before take-off time.
It is a simple matter for a programmer to alter the no-fly blinkie thing on your expensive toy. When you buy one, you can essentially "shop for an opinion" by selecting the no-fly algorithm upon which you are willing to gamble your life. You think computer manufacturers aren't aware of these discussions? That's why they build restrictive programmable tech computers (they're
usually black or made of 3 pounds of of solid brushed aluminum) or they also offer loosey-goosey hockey-pucks with
green-
yellow-
red pixels for us warm-water-pretty-fish dweebs.
It's even caused liability-wary Certifying Agencies to alter their printed materials and training books. Don't think they're under pressure for the travel side?
Like all 180° opposites, let's look at the other end as well.... Is it safe to dive after the dehydration of flying?
Resorts do not want to tell guests that they should fly-in, then sit on shore and hydrate (non alcohol) upon their arrival afternoon. They do not want guests to be denied stepping off the transfer bus from the airport and onto a dive boat. Many guests simply can not wait and oh- what a selling point (story to tell back home). Either that, or there's a welcome rum-punch party. That makes perfect sense.
There is no real study about hydration and DCS either- mostly because it would be monumentally difficult to track. And remember, the "industry" isn't looking for "
wrong" answers~ they want divers.
I have found that many visitors who are concerned with hitting the water immediately upon arrival, or pushing the no-fly limits- these are generally people who should have been more motivated to do every dive offered during their short stay in paradise~ versus sitting out a day because of shopping or a hangover (commonly referred to as a tanning day).