Water is what enables a pliable crust to form and allows the tectonic plate system to operate. This includes subduction zones, along with granitoid intrusions and andesite volcanoes, which maintain the continents.
If the water evaporated, carbon dioxide would degas and the plate system would be replaced by a plume-led dynamic. Earth would be a much different world, like Venus or Mars.
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Seawater constantly sways to imposed rhythms. Surface waters respond to the drag of prevailing winds and the rotation of Earth by forming huge circulatory gyres. Just below are currents that reflect local and seasonal conditions. In the deep are abiding currents which span much of the globe.