I dont think there's any unused space at the bottom of the ocean, we keep the ocean down there. If you fill that space with air bladders you'll have just raised the sea level by whatever amount of space you inflate the bags with.
You are aware, are you not, that there is 3x as much sea floor as land?
And raising the sea level would take more air than has ever been breathed by the entire taxon of primates. You would never store more air than needed for a day or two of power generation.