Far and away the largest amount of ocean controlled by a unified political body is the 3/4ths of the Pacific controlled by Nihofornia's Immediate neighbor, The Republic of Moana*, sometimes called the Kingdom of Oceania, especially in dated public school textbooks. The republic is a constitutional monarchy, which elects a new "monarch" every 10 years or Sooner if the ruling monarch starts falling down on the job. Currently, it is ruled by King Kamehameha the IIVth, who is just starting his second term as Monarch.
*"Moana" means "Ocean". The Disney character's name is "Ocean". :/
The actual size of Moana is up for debate, because while the political borders extend over 8,000 miles from Rapanui (Formerly Easter Island) in the east to Palau in the west, and over 5,000 from the Hawaiian Islands in the north and Definitely as far south as Aoteroa (formerly New Zealand), including the entire South Pacific Triangle centered on Point Nemo, 15 time zones, the equator, and the international date line; some critics (read: whiny babies) try to argue that only the actual landmass counts. Anyone who has tried shipping or moving through Moana will tell you that the ocean ABSOLUTELY counts.
The "Definitely" part refers to the fact that the republic is still growing- After WW3, Moana's borders extended well into what had previously been Melanesia and Micronesia, and into the Coral and Tasman Seas. Currently, there are petitions going around and the beginnings of Legal proceedings for the independent nation of Tasmania to join the Republic, as well as claims on shipping routes that cut through the international waters close to Antartica, which has set the international courts into pearl-clutching mania.
Even more controversial was the not-entirely-joking motion King Kamehamea filed last year that suggested that, as the Republic is extremely large and important in terms of population, land mass (even by the smallest measure), economics and culture, that it was just plain silly that the international date line ran through his country, and that it should be moved to somewhere less populated, like the middle of the Atlantic, or perhaps some small island nation nobody cares about :)