given the respective capacity factors (4,000 full load hours for offshore wind, and around 2,000 for the Hoover Dam, you'd need only 3 or 4 such wind farms to replace the Hoover Dam's net output (I was going to say you needed more, but the capacity factor of hydro can be quite low, depending on location and usage).

You have onshore wind farms which are larger than that in the US, so if you're not hearing about it, it means you're not paying attention! Offshore wind is not yet happening in the US, because it is more expensive than onshore, making it difficult for politicians to put in place the necessary regulatory framework, and NIMBY seems even worse than onshore (cf Cape Wind).

Wind power