What would happen if the oceans dried up?
Probably the end of life as we know it!
This would mean that the water cycle would stop, rain would no longer fall, plants would no longer grow and the entire food web of the planet would collapse. Most life on Earth would die, except for bacteria that might continue to live for millions of years in deep geological groundwater locations and similar formations.
Removing this much mass from the Earth's crust would also likely impact plate tectonics in ways that would be hard to project. But volcanic activity might increase as a result of unloading the crust.
Don Mac is right. The list of disastrous consequences is long
I just wanted to ad a few items on the list, mostly to make us think about how important oceans are.
No more fish is one consequence. Oceans are made with water, which is the only liquid which does not freeze except on a thin surface, due to the reliquefaction of the ice below provoked by the weight of the ice above. This ensures that oceans act as regulators of the climate. They are warmer than the ground in winter, cooler in summer. With no ocean, we would freeze.
With it's mass changed, the earth would be knocked out of its orbit and would end up farther away.