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I want to turn a planet fully covered in ocean into something Earth-like.
How to turn this planet to have islands and continents?
Not answering this exhaustively, but pointing other ways.
You need to get ride of some water (hard) or reshape the surface (easy). If the Earth surface was flat we would have no emerged land too. The cheapest way to do this is using bombs on geological faults if the planet have a molten core. Some bombs on mantle can help too. Pointing a lot of asteroids or moons to the planet to build a mountain is other way.
Getting ride of the water can be done in the physic way, launching it on space, or in the chemical way, breaking it in hydrogen and oxygen and putting these elements in solid substances, using catalysts, atomic robots or any other method. The simplest way to turn water into rocks is to have some free carbon and sulfur and make coal, but the carbon and sulfur in the planet should already be in compounds.
You don't need to do anything nearly as extreme as moving the whole planet to a new orbit and boiling the oceans.
Presuming the underlying geology is already suitable (which moving the planet and boiling the oceans wouldn't fix anyway), you just need to get rid of all the excess water, somehow.
So, just launch it into space. If the planet rotates fast enough, build a space elevator and start pumping water up it. Build a lot of them if you want it to go faster. If the planet doesn't spin fast enough for that, build a Lofstrom launch loop, or an orbit ring, or whatever mass-space-launch infrastructure you prefer. How quickly you get it done depends solely on how much power you can afford to pour into the project--space-based solar arrays would help with that. And there's nothing to prevent you, and the native lifeforms, from continuing to live there throughout the entire process.
Perhaps not entirely Earth-like, but depending on the technology available in your universe/race/story, you could do something similar to an idea taken from The Life of Pi.
Pi Martel encounters a floating, carnivorous algae island, large enough and stable enough to host a colony of Meerkats, as well as himself and a full-grown Bengal tiger. With a little bit of research, we find that this isn't too far-fetched, as stories of natural floating islands DO exist. Here's two notable examples: Cranberry Bog, Keibul Lamjao National Park. There are also explanations of large masses of peat rising from the bottom of swampy areas, stable and thick enough to support shrubs and trees.
So depending on the properties of the plant life on your world, combined with some technology and minimum (if needed) handwavium, a terrestrial ecosystem could be naturally established, or established through man-made means in a semi-natural way.
Edit for explanations: Without more detail on the flora of the planet, here are a few general ideas of how it could be achieved.
A. Masses of entangled plants could be detached from the seabed. Depending on their composition and entanglement pattern, they may need some buoyancy help, or none at all. The rigidity of the floating mass would also help shape architecture of any humanoid life on the planet.
B. Plant life suited for colder environments that has certain features causing it to be buoyant and stable could be brought in from other ecosystems and stimulated in the new environment. Ideally the plants would be able to form some sort of symbiotic relationship with some of the local fauna.
C. Reaching a little more far-fetched here, a species of plant that is known to have rather massive growth could be planted in areas of the ocean where said species of plant will be capable of breaching the surface and fanning out. That species of plant would either need to be capable of growing underwater, or would have to be planted in a specialized man-made environment until it was fully matured. The specialized environment could either be built on-site, and removed once the plant matured, or could be built off-site, and transported and transplanted.
D. Branching out (haha. sorry.) from plants towards my most far-fetched idea is floating sponge islands. To maintain the buoyancy, the humanoid race that I assume is going to be living there could run a system of pipes through the sponge, drawing water out of the sponge layer and eject it back into the ocean. Since I don't know much about your planet's location relevant to any nearby star or it's atmosphere, I'm going to assume it's cold due to lack of sunlight, so you'll have to reconcile the obvious power costs of such a system somehow.
Hope these help!
Any planetary engineering of this sort is going to require a massive amount of advanced technology and powers that are many orders of magnitude beyond what we currently have. So it depends on what sort of powers are available. Assuming very capable advanced hi-tech, it might work like this:
Take breeding colonies of all the life forms you want to keep and store them off planet somewhere safe. Next move your planet into a very close orbit around the sun and allow it to cook in the heat. Before long it should be converted into a Venus like world. Keep the world boiling away until lots of the oceans have boiled away and a lot of the atmosphere has been blasted off by the sun.
You must be careful not to overdo it, but with advanced tech it should be possible to determine when it’s “done”. At that point move it back out into a more distant orbit to cool off. If you’ve judged it right there will be a lot less water now but still enough to form Earth like oceans. Bombard the planet with a whole load of comets to help trigger plate tectonics and wait for it to settle down.
Reintroduce your living creatures, starting with those capable of producing oxygen, allow oxygen to build up and then introduce the rest. Alternatively just introduce whatever life you want to take hold
Just some thoughts.
1) Pump the water to the poles and freeze it there. Ice is rather reflective so it would keep itself from melting for some time. You'll have to worry about possible cough climate change cough in this case.
2) Genetically engineer algae to produce buoyant organic remnants. There must be a lot of CO2 stored in this huge ocean, enough to produce "platforms" capable of sustaining a civilization. You'll have to dive for metals/minerals, though.
And also there can be methane/oil captured on the seabed. This can be turned to floating substance, too.
Everyone is saying to get rid of the water.
This is incorrect.
Our lovely Earth (particularly its dry land) could be entirely submerged by the water existing upon it, were it not mountainous and trenched. (If all land was equidistant from the centre.)
All you need is tectonic activity, subduction, lift, convergence, volcanism. Rifting and trenching. You will generate deeper oceans to hold the (same amount of) water, and uplifts which become the dry land.
