A small interjection: the flat-topped seamounts or guyots of the Pacific are, indeed, apparently old volcanic islands whose flat tops were once at sea level. Some were atolls - fossil shallow-water coral has been cored from them. But there is a very non-WaltBrownian explanation for why these structures are at the depths that they are.
They were once volcanos, so they sat atop hot mantle and crust - they had to have magma to be volcanos, right, Charley? But their hot connection cooled, and the rock they sit upon contracted with that cooling. The same shrinkage is responsible for the shape of the Atlantic Ocean floor - there's a ridge at the hot middle, and then the seafloor is deeper under water to either side of the ridge. Then the edges of the Atlantic abyss get shallower again due to sediment accumulation.