The Guardian reports a study that shows that about 300 million people live on land that will be flooded by 2050 as a result of rising sea levels. In comparison the US population was about 327 million in 2018.
Since it seems very unlikely that effective measures to combat climate change will be implemented anytime soon (probably never), we need to relocate an amount of people slightly less than the entire US population.
Given that there is a worldwide anti-immigration sentiment, it seems difficult to find suitable homes for all those people. A rather small portion of those refugees could become seasteaders but it is rather questionable we can house hundreds of millions people at sea.
So unless effective anti-climate change measures are implemented, the construction of space habitats in Near-Earth Space might be the only solution.