People often think of terraforming planets as one of the most viable options for humans colonizing space. Or they think we will arrive on some planet that has life already, and just start living there. Science fiction, especially shows like Star Trek, are replete with the idea that Humans, Vulcans, Klingons, and whatever could just casually interact with each other, and even mate with each other without producing serious biological consequences. This is totally absurd, since even a small bacteria carried by aliens could devastate the planet Earth's biosphere. Imagine if aliens arrived on earth and transmitted a bacteria to the surface of our planet that converted all the atmospheric nitrogen to nitrous oxide, destroying the ozone layer, or a bacteria that turns sea water into hydrogen peroxide, bleaching all life in the oceans. Merely allowing an alien species to land on the planet might transmit an apocalyptic bacteria to Earth. The tolerant world of Star Trek, where multiple species live together on planet Earth, is a fairy tale.
We have two foolish notions here; the belief in cohabitation of biospheres, and the idea of terraforming itself.
A far more efficient way than terraforming is to modify humans to fit the environment of another planet through sophisticated gene modification. Many extrasolar worlds may turn out to be ocean planets, like Gliese 1214 b is thought to be. Assuming that such a planet is devoid of life, the easiest approach would be to genetically modify human beings to have gills and breathe underwater. Of course such humans would also need different skin that can tolerate constant exposure to salt water, sonar to communicate under water, collapsible lungs, different eyes, and a few modifications to the brain to process sonar signals. A sufficiently modified group of humans could colonize a water world this way. The moons of Europa and Ganymede are already water worlds right now, and if early indications are any measure, most Earth-like planets will be water worlds. This is because Earth is an outlier. The Earth was formed when a Mars-sized planet smashed into it. This created both the Earth and its enormous moon. Without this event erosion might have ended plate tectonics by now, turning the continents into enormous sandbars. If the Earth had been larger it would have retained more hydrogen, and hydrogen and oxygen make water. A larger planet would therefore probably be covered in an ocean miles thick. If the Earth were smaller it would probably have lost most its atmosphere to space. Having a medium-sized planet, large moon, active plate tectonics, and thick enough atmosphere, is a remarkable coincidence of exact conditions. Most of the planets in the galaxy that have water are unlikely to meet more than one of these conditions.
Additionally, there is the issue of gravity. A larger planet will have a stronger gravity at the surface. Interstellar travel will involve a substantial amount of bone loss from prolonged weightlessness. For humans to travel in weightlessness to another planet, spending years or even centuries in space, and then land on a planet with an even stronger gravity than Earth, presents a huge logistical challenge. The issue of gravity is almost totally solved by having human beings emerged in water, which acts very much like a weightless medium on the human body.
Most planets have radiation problems owing to the absence of strong magnetic fields, and moons that orbit gas giants are saturated within the radiation fields of the gas giants they orbit. Water has an excellent ability to shield things from radiation, which is one of the reasons it is used in nuclear power plants. Even if an ocean planet lacked a magnetic field it might be possible to simply live under the ocean at a few hundred meters depth, and be shielded from most forms of radiation. Humans with gills to breathe could simply build habitats under the water, and use their lungs only when they came to the surface. This is a far more valuable way to live than trying to massively alter the environment of a radiation soaked planet like Mars, or a CO2/sulfuric acid atmosphere planet like Venus. On these other worlds, regular humans would need to mine and carry around oxygen at all times. That is dangerous. Imagine trying to force a baby to always wear an oxygen mask outdoors. On Mars they would still need to live under the surface because of radiation, and on Venus they would have to build floating habitats, and contend with sulfuric acid. Again, this is dangerous for raising children. In contrast, a genetically modified person living in an ocean planet could simply breathe anywhere they went by making sure water was flowing past their gills. Even if the water was extremely cold humans might be engineered with Antifreeze proteins (AFPs) in their blood, like Antarctic notothenioid fish. Also, a vast abundance of water solves countless problems with farming and getting rocket fuel, and habitats can be stabilized against sinking by simply splitting the water into hydrogen and oxygen with electrolysis, and using the oxygen to form a bubble at the top of your habitat to counteract the weight of the structure. A sphere-shaped habitat could be floated under the water with a small amount of oxygen for lifting gas, and the hydrogen could be used as rocket fuel. If there were any hurricanes you would simply submerge the habitat to a depth where the water was calm.
On top of this, there are advantages to having a greater pressure outside than inside. Bubbles are easy to identify visually, and so leaks can be found and repaired, and gas escapes more slowly. There is also a smaller pressure difference, eliminating the possibility of explosive decompression.
As a first step to developing this genetic technology one could begin by genetically modifying pigs to have gills. Pigs are close enough to humans genetically so that human organs can be grown inside them, and that means they make a great initial test subject in case anything goes wrong. You want to perfect the technique in animals before you try it in people. Since there is no reason not to take these farm animals to the ocean planet with you, you wind up bringing aqua pigs into outer space.
(I just like saying that phrase).
You do not actually need to give humans flippers or anything like that. They are already going to have grey-colored skin (genes borrowed from a dolphin), sonar, weird eyes, and gills. They can use conventional plastic flippers to swim though the water, and they will need their opposable thumbs to continue to work with technology. Furthermore, the spaceships they build may be filled with water rather than air, or have a combination of water for working spaces, and air for sleeping spaces. Remember that a human with gills must move around in order to breathe. This means they will need some kind of fan blowing water past their necks to sleep under water, or an air-filled chamber where they can use their lungs. They may also want to eat and defecate in an air-filled room for the sake of hygiene. A spaceship for aqua-humans will probably have both types of rooms in it. Some things are easier to make in air than water, and some combustible things might be made in the water for safety.
The point of all of this is that changing humans to live under water is a far more viable long-term colonization plan than making planets habitable. There already are water worlds in this solar system. Water also slows bullets and rockets fired by enemies, and acts as a natural shield. Aquatic farms can be constructed which grow algae, sea weed, fish, shrimp, and vegetables genetically modified to live in salt water. It is far easier to genetically modify a living organism to tolerate salt water than to genetically modify it to survive the lack of water and sulfuric acid of Venus, the radiation and dust storms of Mars, the liquid methane lakes of Titan, or any other horrible environments. We are genetically adapting Earth life after all, and the more Earth-like the planet the easier it is to adapt it.
Yet another great benefit of this approach is that it can be entirely perfected on Earth before even bothering with colonization. A probe can be dispatched to drill under the ice of an ice world and identify if there is a warm interior heated by tectonic activity. If so, and identical part of the ocean can be located where the pressure and temperature are the same. A base can be built there to train astronauts. Genetically engineered humans can get used to living in these environments long before any colonization mission is attempted. When they arrive the conditions they experience will be virtually identical to the ones they are already used to.
Last but not least, the development of the technology necessary for accomplishing this also creates a method of exit on Earth, and since most of the world is covered in ocean it opens up a frontier for freedom here. Imagine hundreds of underwater cities on this planet where, unlike the video game Bioshock, all of the people simply swim around in wetsuits under water with no need of oxygen tanks. They live in spherical houses stacked more vertically than horizontally. Their houses contain "water conditioning" that uses the compression cycle of a refrigerator to heat the house while cooling refrigerated sea food. The house blows water continuously around each room so that oxygenated water always flows past the gills of the inhabitants, who literally live in water at all times. They speak a combination of English and a made-up sonar language. There is a bubble of air in the "attic" of each spherical dwelling where you can surface, use your lungs, and converse in normal language. There are air chambers for doing business with regular flatlander humans, and people use sea scooters and other propeller-driven flying watercraft to commute to work in vast schools of aqua sapiens. Everything they manufacture is designed to work in water.
Imagine a man who looks like the Greek god Poseidon, with a beard, grey skin, and gills on his neck, chilling at home in his living room, and wearing a wetsuit at all times. He keeps his house at 89 degrees Fahrenheit. He lives this way, and breathing air is for weird terra sapiens. This dude has never even learned how to walk. Walking is for weirdos.