What would happen if the Ocean became one big dead zone?

As we know the amount of dead zones (a part of the Ocean with extremely low oxygen levels making it inhabitable) are increasing, so what would happen if the ocean became one big dead zone?
Update: I was wondering how and why we'd all die, would it be because we'd have no water? Or because the plankton could no longer help rid excess oxygen? Or all of these things.
Sorry for not being more specific
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