Life On An Ocean Wave

The population of the planet is 7 billion and growing and as more areas become deserts, flood plains or so polluted that it will endanger human life to live there, the space available on the remaining 29% of the globe that isn't water is going to become pretty cramped.
We have a few options such as using the space above our head or the ground beneath our feet but building massive space stations are beyond us and living underground has the obvious problems of lack of sunlight and ventilation so the only option left is to build in the sea.   
We are already building structures in the oceans such as oil rigs, forts and tunnels and there are several man-made islands off the coast of Dubai so we have the technology and we have the sea so why are we not doing it?
The Pacific Ocean is 65 million square miles which is equivalent to nine Russia's. We could build three islands the size of the three largest countries slap bang in the middle of the 30 million square mile Atlantic Ocean and still have over 17 million square miles of Atlantic left for the fish to do there thing.   
A new Britain could be piled up on the other side of Ireland and as the old one holds over 65 million people that would relieve all of the overcrowding we face here in our cities.
The obvious problems would be material and the expense but a bit of levelling of the landscape would provide the material but coming up with the money might be a bit tricky and the last thing the world needs is a new country that is privately owned by a company.
It's a thought though and as more and more people are forced to live on areas that were previously deemed undesirable through lack of space, it might become something that we have to consider because we can't go up or down, only out.