Why is there a mountain system in all oceans?

Answer 1

Mountains arefound in all the oceans because of uplift or thrust from below. Oceans represent thinner crusts of the earth, by elevation land under the sea is lower than land heights of the continents

Answer 2

For the same reason there are mountains above the ocean; being covered in water is incidental. The earth is covered in tectonic plates, large masses of land, the size of whole countries or bigger, that are constantly moving around; bumping into each other and sliding apart. This movement creates mountains over a long period of time, also, valleys and the general shape and 'lie' of the land. This happens all over the earth; above water and below it, since tectonic plates are below everything else.