why is sea / oceans water not fit for domestic use? write in short.

The amount of salt makes washing clothes infeasible because the salt would be left behind when the clothes dried, the salt crystals are actually sharp enough to wear at the fibers, it would also feel like sandpaper on your skin.

The technology needed to treat ocean water used to carry sewerage would be extremely energy demanding and would wear out the equipment a lot fasted than freshwater. Saltwater is pretty corosive.