There's a lot less of that very-distant ocean than you might think. You need about 700 km of lateral distance on Earth to get 800km distant from a point 400km over the surface.

Start drawing 700km bubbles from random specks of land in the ocean, and the uncovered ocean disappears quite quickly. Bermuda, the Azores, and Newfoundland conspire much of the North Atlantic between the US and Europe. Seychelles, the Maldives, and the British Indian Overseas Territories similar get a decent coverage of the eastern Indian Ocean. Random Polynesian islands also manage to provide a surprisingly good coverage of the South Pacific (almost contiguous all the way to Hawaii), although the gaps are more evident here.

I don't have hard numbers here, but I'd hazard a guess that the ISS is closer to civilization about 80% of the time--not bad, considering that civilization occupies far less than 30% of the surface.