Just a bit of clarification, you're referring to capacity factor, not availability.  Availability is the amount of time a wind turbine is available to operate, completely independent of whether the wind is blowing or not.    Current wind turbines aim for availability factors of up to 97%, and rarely reach that, though are often above 92% and higher.  Newer turbines usually struggle to reach 95%, and availability is often a function of how much extra service attention a machine gets.

Capacity factor is the percentage of full load achieved over X time, usually a year.  No plant, including base load, achieves 100% capacity factor, in fact, many base loads are in the 70's (though some reach in the 90's).  Nuclear power, for example, often produces capacity factors of 100% while they're operating, but of course like any plant, 0% while they're shut down.  In fact, in the US, nuclear power capacity factors have just reached fractions above 90% in two years since the early 70's, and did not reach above 80% until 1999.  (EIA)

According to the Nuclear Energy Institute, the average capacity factor in 2006 was 89.9%, compared to coal at 71.1% and nat gas ranging from 17.2% to 39.9%.  (This is because coal plants need significant maintenance downtime, while natgas is burned primarily in peakers.)

No question offshore has higher potential capacity factors than on land, because the wind is both stronger and more consistent.  What's also most advantageous about offshore is the lower turbulence intensity. (TI is a wind turbine's enemy number one.)  This is because there's nothing to increase surface roughness for thousands of kilometers, except waves and more waves.  On land turbulence is increased because of terrain changes like hills and valleys, and buildings and bridges, and cell phone towers and carnival rides, and the odd wind turbine here and there.

"Life shrinks or expands in proportion to one's courage." - Anaïs Nin