Time to take an in depth look at the phenomenon of flying mammals. Together we shall discuss this from a science perspective and add all our thoughts together till we have rationalized these glorious creatures. To start I will warn the reader, there will be talks of whale fecal matter and farting so squeamish folk may now exit the thread.
And now we begin...
The sky whale has been depicted in various art forms as a graceful baleen whale dancing in the clouds with humans looking up in awe while the whale looks down and thinks..."Ha puny humans, I own you! Who's hunting who now?!! Muahahaha!!"
So lets take a look at some of the prerequisites for these creatures existing in GOIO:
-Stable food source in the sky
-Movement and flight capabilities
-Waste disposal...there will be pictures!!
-Reproduction time
-Death and predators
-Feel free to add more...
Let us begin...
Food. Food is problematic when we consider the artistic concept of the Sky Whale being a baleen whale.
These species of whales have to eat tons of microscopic organisms each day just to sustain their bodies in the ocean. So how could they do this in GOIO unless...the skies were filled with flying krill. Now if that is the case then consider how much flying krill it would take.
Literally, we shouldn't be flying through white puffy clouds but through clouds of colorful microorganisms. Which brings in the next inherent danger. Whales feeding on krill clouds when a ship passes through them.
The age old fear of being swallowed by a whale is actually not as scary as it seems. See the whale has a giant mouth but a rather small throat. There is a good chance you would survive the encounter. If you didn't, then you could rest easily that you wouldn't be digested but your body only mangled and probably tongue lashed till your body is completely crushed. If of course the creature doesn't realize you are in there.
Now Sky Whales seem to be on a more massive scale so lets consider that if the mouth was say, large enough to contain a Galleon, then most likely your ship wouldn't be swallowed whole. It would likely be smashed and crushed to some extent. Anyone not strapped on would be then swallowed immediately, but your chance of survival would be higher. At least until it spits you back out and you plummet to your death because your balloons are completely gone.
Smaller ships, yeah, you're just lunch.
Now if we throw out the baleen whale version of Sky Whales then we have toothed whales flying through the skies. This is actually a bit more plausible but they would still need flying squid or a large food source so that would have to be rectified.
However, the pleasant image of a whale in flight is forever destroyed as you see a toothed whale in flight, bearing down on you with a hungry look in it's eye. So for them, we can consider humans and ships on the menu.
So keeping that in mind, since we obviously aren't flying through krill clouds or running for our lives from Sky Whilly. Lets look at an alternate to Sky Whale feeding.
Sky Whales, don't start out as Sky Whales. This is in turn brought on by something they eat in the oceans of GOIO.
Now if we consider what would cause them to become Sky Whales then we have to consider digestive systems and farting. Our farts are actually made up more of hydrogen than of other gasses thanks to our glorious bacteria which marches on breaking down our rubbish so we can expel it later as brown gold.
So something a Sky Whale eats would cause it to amass hydrogen in great quantities. It would have to do this in such a way that it would reach a critical build up of gas which would cause the whale to lift out of the water. This means, no whale pooping in the water before it takes flight...
So we have our mysterious wonder hydrogen krill which simultaneously inflates whales till they can fly, and at the same time closes off their butt so they can't expel the gas. Now lets look at their graceful flight...
Simply put, if you've ever eaten something really bad and been stuck on the toilet pooping for hours and your stomach just won't settle...imagine being a whale with that problem and then gracefully flying through the crowds looking cockily at humans.
Yeah, not happening. Skipping aside the concept of how much the whale's body could take before it just tears itself open due to gravity and pressure changes. You have the graceful image of a bloated, constipated marine mammal floating upside down lifelessly or barely twitching. It's only thoughts, not of pride on humans but begging for someone to shoot it and put it out of it's misery.
The end of the poor creature would be the eventual massive whale fart, poop, that would cover the sky and expel the gas...or the eventual explosion as the hydrogen being volatile will likely erupt raining guts either over the ocean or where ever the poor thing manages to drift over.
But lets say this creature is gifted with a special gas sack which allows it to amass the gas at will and expel it whenever it wants? Well, then we have a different creature than the artist representations. One which would need multiple sacks to be able to balance. Also one with more aerodynamic flippers to specialize it for flight instead of water.
If these sacks and distribute gas throughout it's body, then they need a way of expelling gas also. So these creatures would literally be constantly farting as they change altitude or maneuver. Those glorious artistic images would have to replace fluffy clouds with brown patches. This would in turn likely work against global climates as it would be like having all the cows owned by McDonalds in the Amazon rainforest, farting in unison.
Reproduction would in turn require very precise maneuvers as wind currents could easily send a whale from "wooohooo!" to "Holy crap I think I broke something!" Unless the creatures are born with some form of grappling apparatus or claws, it just isn't going to be pretty. Certainly not worthy of artistic renditions of "Daddy, whats that whale doing to that other whale over our house?"
Hence the whales in theory would work better if they exist in both water and sky environments. In water they live and in the sky they...explode?
Which lastly leads into death and predators. We've already covered the explosion possibilities with giant flying mammals but what about predators? Likely these would be other whales. Possibly if there was two species. One a baleen whale variety and the other a toothed whale variety. The other possibility is...Sky Sharks...but that is another discussion and one that only ends in lasers.
The whales themselves would otherwise have to live out extreme long lives. If not, the landscape would be littered with crushed cities and corpses. The things would literally just be falling out of the sky. Specially when they blow up. Cloudy with a chance of entrails anyone?
Now there is a great plus side to these flying exploding beasts of flatulence. They'd make great fertilizer! If you, happen to survive to that point without being eaten, pooped on, or crushed under tons of whale girth. Oh and then theres the explosions....