Not because we are running out of land.
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. There is enough food and enough space for everyone to live - on land itself. The problem is that the food and space is not evenly distributed. There is enough food for everyone, except that not all are empowered equally to earn their fair share of it. We have made enough progress to produce enough food for everyone, but we haven't done enough to make sure it reaches to everyone who need it. I use food as representative term for basic material needs of human being, rather than literally.
Providing for the large population is a technological problem. Making sure the produce reaches everyone is socio-economic-political problem.
Let's see how we find technological solutions and see if we can apply the same steps to solve the problem of everyone getting what they need.
In technology, we first identify the problem. From the analysis of the problem and our available knowledge of science we come up with a solution. We build a prototype to test if it solves the problem. If it doesn't we improvise and iterate, until we build something that will solve the problem.
This is the recipe for every man-made object. Be it flying like a bird, or finding cure for Malaria - all technological solutions follow the same pattern - analyse and experiment.
So how do we apply the same process to the problem of creating a society where everyone gets their fair share of resources. We have already identified the problems that keep us from creating such a society. Poor people don't get enough, because they don't have money to buy it, because they don't have jobs to earn money, because they don't have skills to get jobs, because they didn't receive any education to build skills, because their parents didn't plan how they will provide for their children before giving them birth, because they didn't have education and sense of responsibility themselves. We know all the problems that keep the people below poverty line from rising above it. So the next step towards finding a solution is to do experiments. Try different policies - make education mandatory, enforce birth control, enforce minimum age before having first child, change retirement age. There are many social and economic experiments that we can perform that will help us finding the solution. So what's the problem? The problem is we cannot do these experiments as cheaply as we can do technological experiments.
Before we can try out some radical social or economic policy it has to go through Government approval. Government is a democratically elected entity, which is how it should be, but that makes it unlikely to do experiments that may have a chance of success, but will displease a majority of their voters.
So where does sea steading come into picture in all of this?
We need sea-steading to create societies that can do these experiments. There is nothing special about sea, except that we have run out of land where like minded people can gather and just start a new country with new rules. All the terra has been filed as a territory under jurisdiction of one government or another.