Re: So is this just silly
Presumably there is a planet there below them, though?
Was there land once, but they had to escape some disaster and took to the skies in man-made (sorry person-made) islands or is this a natural phenomenon? If the latter, how deep are these islands? A lot of Earth's mineral wealth is found by digging - are you going to run the risk that someone might tunnel out the other side?
Man seems to have originated in one place and spread by mass emigration, taking to the seas to get to different continents. Animals, too, would have found their way to different islands by walking across now-submerged land bridges or swimming narrow straits - even surviving long crossings by luck. Your peoples and their animals aren't going to be able to do that - producing some kind of rudimentary boat is one thing for a primitive civilisation, but flying machines...? That means that for practically all of their existence your peoples would be marooned on one island. Even allowing for it to be a big one, that brings with it problems of development - societal, economic, intellectual. Are they ever going to develop sufficiently to produce craft to get to the other islands??
And how far above the planet will they be floating? The higher you go, the colder it gets - so if you are planning on having vegetation on these islands to feed the animals which evolve into your peoples, you are going to have to think hard about what kind of plants are likely to grow there. The less vegetation, the fewer animals, the less chance of evolution producing sentient creatures, the harder it is for any such creatures to breed in sufficient numbers to throw up the geniuses needed to make developmental steps in civilisation - and the easier it is for in-breeding to create a race which goes nowhere.
It would be warmer inside the islands I suppose, but then you've got issues over lack of photosynthesis - plants and animals can survive in caves without ever seeing daylight, but you're back to numbers and evolution having a hard time again.
If the islands are only a few thousand feet up (and that can still get pretty cold and barren) then your peoples will easily be able to see the planet below them - it's still a very, very nasty drop but it's not endless. And people being as stupid as they are, surely from the very beginning they would be trying to get down there all the while, to explore - creating a very long rope to go down is a damn sight easier than building a plane to get to another floating island. Even if there is no land, presumably there is something - liquid perhaps. Unless it is corrosive, wouldn't it be as easy for them to build boats to live on the liquid. No crops down there, but still the possibility of creatures in the liquid which they could hunt.
And you thought your only worry was making sure they didn't fall off!!
Sorry. As you can guess I like to follow things through - it makes reading hell sometimes as I sit and nit pick away. Luckily I'm not any kind of scientist otherwise I'd never be able to pick up a SF book without making lists of all the things that are wrong!
Please don't let me stop you going ahead with your idea - and let's face it, if it's fantasy you can re-write whatever laws of physics you want! - but I think it might help if you spent some time brainstorming how the people have evolved and how their society has developed because that will inform their spiritual beliefs which will have an impact on how they relate to each other, particularly those who are different (eg those who have made a life on the liquid, or those who have separately developed on a smaller island which split away from a larger one centuries beforehand) - giving you conflict that might be helpful in your plotting.
And when you make your first million from your best-selling range of floating islands books, make sure I'm remembered!!