Why hasn't man conquered the seas yet? I mean like massive, floating continents. I know the oceans are one hell of a beast but is it possible to build a floating structure so large it would be inpervious to capsizing? Keep in mind the benefits of the sea:

Less disease/insects
Unlimited power due to currents
Unlimited source of purifiable water
Good source of food if you suppliment it with farming
Less pollution

Other than the obvious starting, engineering and maintenance costs (salt water corrodes like a bitch) am I missing something there? Seems like a huge untapped resource to me.