So I'm wondering if it would be possible for humans to live healthily on a largely Earth-like planet that was covered almost entirely in salt water. Landmasses would all be islands, with only a few reaching sizes of, say, 300,000 km$^2$, and the rest being around half of that or less. These landmasses would mostly appear in groups, with only two or three of these groups comprising most of the land surface on the planet. The islands would be varied ecologically, but the variety of habitats would be similar to Earth (regarding plants, animals, environments, etc). All in all, I don't imagine more than about 2% of the entire planetary surface would be land.
I'm mainly wondering what the atmospheric implications would be, i.e., would humans be able to inhabit these islands in large populations (at least so far as natural resources allowed) and actually breathe? Obviously, there would be far fewer trees given the lack of land, so there would be less oxygen produced via photosynthesis. But if the landmasses were grouped closely together, thus lending their collective oxygen output to each other (if that's how it works?), and there were plenty of trees on many of these islands, and there was a substantial amount of marine plants such as phytoplankton in the oceans producing their own oxygen (they already produce around 70% of the Earth's oxygen), would humans be able to flourish?