Offshore wind power costs ~3 times more per MWh than on land. You can still do it if you're really low on space, but if you have tons of empty space anyway, it may very well be cheaper to simply build longer power lines.
The main advantage of offshore power is the reliability of the wind. For example, on the North Sea the wind blows strong enough for power generation 80% of the time.
The East coast of the US has similar trade winds.