Do you think swimming windmills will be viable some day?

If not, and only the continental shelves can be utilised, they won't add that much in the case of the USA (unlike in the case of Europe). Consider this: utilising the entire continent (not taking into account limitations in zoning laws) and shallow waters, the US total capacity with present technology was estimated at three times the current consumption a few years ago. Incidentally, another study estimating the electricity need if all US cars go fuel cell also put the need at three times the current usage (i.e. the new need would be four times of today's in total).

There is not much benefit from size: you have to place larger wind turbines further apart. As both the power of a wind turbine is proportional to the swept area (hence the square of its rotor diameter) and the required distance between rotors is proportional to the rotor diameter (hence the number of rotors on a given area is inversely proportional to the square of the rotor diameter), the two factors cancel out. What you can win with size is some economies of scale (less maintenance or installation costs), but it can break down with further increase in size.

_{*Lunatic*, n.
One whose delusions are out of fashion.}