So my current book set in distant future on earth where earth surface only covered with oceans with no more land mass ( kinda like a water world ) and the only lands are floating islands that made from shattered continent. It would be easier if I make this setting on another earth like planet but I want it to be earth with the previous civilization left burried in the deep sea. My question would be what kind of castratophe that could make this scenario. Since my genre is a fantasy I don't really need things to be scientifically accurate only need it to believeable.
Your ancient ancestors (read our descendants) saw the coming apocalypse in the form of the melting of the polar ice caps. They knew the oceans were going to over run over the land. They had recently discovered advanced forms of energy. They knew far more advanced forms of mining. They carved enormous land masses from the existing continents and fashioned large equipment to it to help keep it stable and afloat.
These land masses were mostly undertaken by the individual nations of earth which now leaves you with a geopolitical climate that you can exploit for your story. Especially because the nature of floating islands would lead to more xenophobic societies.
Fuel/Energy sources might be advanced but are not necessarily limitless. The impending doom of supply failure adds a tremendous degree of tension and motivation to overcome the xenophobia.
Because magic is involved you could substitute the scientific advance and energy supplies with magic power.
I am not sure if you want your islands to float in the air or in the water. If you want them to float in the water, then you need floating bodies. So when sea levels were rising, people made their own floating islands out of old plastic bottles, just like the guy in this video here:
When you are floating around with only your family and very limited space, it would be nice to meet other floaters and maybe you decide to permanently connect your islands. By this process big floating islands could form. They would not be static like real islands, instead you would feel the waves under the island(s) when you go from one tiny island to the next over some kind of bridge.
Of course storms would endanger these islands, as they could rip them apart, so a lot of rebuilding would have to be done every now and then.
The good news is that there is enough water to cover the earth from pole to pole. There are just a lot of mountain ranges that stick out of the water.
There are two ways about it:
Flatten every thing And you will end up with only water on top.
Of course, you will want to mix these up. Shall we start?
Extracts from the Book of the Ocean.
With the population of Earth reaching 15 billion in 2100 we will need more food. A lot more food. We need more arable land, to grow food. So say to the mountains: be flat and grow food! Say to the deserts: be green and thrive! Say to the seas: be shallow and produce fish! And so it was done and men did not starve. And man finally conquered Earth and did with it as it wished.
Then came the rain. It rained comets. First the satellites went down. No big problem, we have backup systems. But Mars and Venus were beyond our reach for now. Even Luna was no longer a travel destination. Those colonies would have to fend for themself.
Then it kept raining comets. And the sea started rising. Levies were put up. And make bigger every year. But then came the storms. With no mountains to break up storms they could and did rage longer with more destruction. So the levies broke and whole countries were swallowed by the sea. Some unlucky countries like Bangladesh disappearing in one big storm.
Some countries started to build shelters that could thrive under the water. Some shelters made it, others perished.
And lastly there were people who loved the ocean and wished to be on her as long as they could. The first Ocean community lived in a converted container ship. It showed the world a way forward. From there on ships became bigger and bigger. The last step was modular "Ships" where you could add pieces as you liked. In the end these were no longer ships as we know them. The are ever changing-islands in a world covering Ocean. Not stationary but not fast either.
By the year 2200 Billions had died, but man still made Earth it's home. No longer blue, green and white, now it was blue and white only. No longer terra firma, people moved with the Ocean, She that takes and gives.
Materials could be anything really. If you have the time you might even use concrete to build these islands. In the end you will want something that can handle salt water and probably is easily repairable.
You will need trade for materials, but you have the colonies under the water for that. Add some submarines for transport and you are good.
Don't get to much into details, just keep it real big picture with some personal stories for people to attach to.