I think I read somewhre Giants are form Atmora and they traveled to Skyrim with the Atmorans. Is this true? And if so? How did the big dumb giants ride on boats with atmorans?

To me it's obvious that giants and men were once the same race, but perhaps giant splintered away. Then again, this is the Elder Scrolls, reality's only an obstacle to be thrown to the side.

Either the giants and Men both came from Atmora, or the men came to Atmora and a group split away and became the giants.