Floating settlements

As I walked on the muddy estuarine looking at crabs scuttling into the marshy,brackish soil,My son picked up a hermit crab ,its legs propelling it forward as it dragged a conical shell along.Ah,would a day come when the soil is so contaminated and the space on land so crowded that we have to build floating skyscapers on floating platforms near the coast.Drinking water could be supplied by desalination equipment on a floating platform and sewage water treated before being discharged back into the sea.That would presumably free up land needed to build dams,divert rivers,reservoirs,complex system of underground pipes and sewage.At the same time ,electricity would be generated by solar or wind or wave power. 3/4 of this planet comprises water so people could stay offshore and be transported to work through underground public trains to work on land.
Materials used must be biodegradable so that it decomposes when thrown into the sea or it could be treated and processed into cellulose materials for growing medium for plants.Fish farms would be set up to produce food for the settlements.School and hospitals need not be physical structures as tele-education and robots be used for minor surgeries.