For decades now, people have feared that the explosion of global population is going to harm the planet because we won't have enough space for all of them to live and not enough food to feed them. The population of the planet is now about 7 billion (A number that some scifi authors foresaw to happen when we inhabited entire galaxy). Yet there is enough food and enough space for everyone to live - on land itself. The only problem is that, the food and space is not distributed evenly. The problem is not about producing enough food, but to empower all individuals of the planet to earn their fair share of it. The problem of growing population has less to do with technology, and more to do with social sciences.
We do not need sea-steading because we are running out of space on land. We need sea-steading because there is no space left on land to do experiments that will build a better society.
In any scientific field, when we want to test our theories about new inventions, we do experiments. If you have an idea for a radical drug that will cure cancer then you won't be able to test it on real patients right away. Because there are laws that restrain you from doing tests of unproven drugs on humans. Instead, you first try it on rats and if things prove promising then after several years of research the refined drug is sanctioned for human testing. If you have an idea for self driving car, then you don't let your prototype loose on crowded streets. You take your prototypes in deserts for experiments. Only after years of testing in deserts, can someone drive a semi-autonomous Prius on streets of Mountain View under manual supervision. For testing radical world-changing ideas we need a test environment. This is as true for socio-economic experiments as it is for technological ones.
For successfully tackling the exploding population of the world, we need to do radical social and economic experiments. At present we try to find the solutions only in the framework that is allowed by the laws of Governments. Governments are democratically elected entities. They enforce the policies that only masses can approve. Radical and world-changing ideas are by definition not agreeable by consensus. Therefore if you have some social policy - like free health care to everyone, social security based on carbon footprint, or a variant of capitalist economy that won't lead to Wall Street - you cannot try it.
In 1947, India became an independent nation. At the time, it didn't have a clue what kind of economic policy it wanted to execute. The world had two strong economic philosophies - Socialism and Capitalism. Both of them were unproven at the time. India implemented a mixed policy - allowing private sector industries, but keeping the Government stake in major infrastructure projects. In 1980s, with the fall of Soviet Union, it became clear a Socialist economic policy with total Government control is doomed. Therefore in the 90s, India (among other countries) opened its policies gradually. Letting private sector and multinational investments in major infrastructure projects. Countries learn from each other for creating their social and economic policies. If one country can execute a social experiment and prove that it works, other Governments will be able to convince their population that it works and eventually adopt the policies themselves.
Better solutions can only be found if one can experiment.
Now that populations are growing and are connecting more closely, they enforce the consensus more effectively. In many obvious cases of social welfare, it works greatly. But due to massive public opinion, even the greatest leaders in the Office think twice before implementing a radical change in nation's policies.
If we want to develop better policies, we have to be able to start from scratch. We need petri dishes to test our experiments. There remains no land to create these petri dishes, so we need to go on water. We need to build cities and towns on water where we can implement new laws and rules. Not all of them will be successful, but they will give the terrestrial nations a data point to correct their policies.