[nicktelford]

As other posters have mentioned, offshore is significantly more economical for a number of reasons (stronger, more consistent wind; large tracts of available area for large-scale farms; the capability to build turbines much taller where they can access stronger winds) - so instead I'll mention another problem facing offshore turbines other than corrosion: the air.

It turns out that offshore, the air contains a lot of minerals and water droplets that make it much more abrasive than the air over land. This is a particular problem for turbine blades, as their leading edge is moving through the air at a significant speed: enough to gradually ablate away the leading edge material! Consequently, offshore turbine blades need to be replaced more often than onshore turbine blades. This is one of the reasons that larger turbines are more economical: fewer larger turbines mean fewer, larger blades that need to be periodically replaced.