What are the advantages / disadvantages of having wind farms offshore?

One of the principle advantages of building a wind farm offshore, is that no land is used for the construction. So there is no requirement to compulsorily purchase land for the project.

Another advantage, is that a wind farm sited offshore is free from geological features, that can obstruct airflows at the site.

On the downside, the construction costs are higher, than they would be for a land site, material constraints are tighter, also the infrastructure and maintenance costs are higher.

In addition to the advantages listed by blacksmith81, there is less chance of the Not In My Backyard mentality holding up or preventing development of windfarms. Also, the landscape, but not necessarily seascape, is unaffected. At sea I guess there is the risk of a catastrophic collision between between ships and one or more towers - on land the chances of accidental damage are probably far less. The phrase 'if it can happen it will happen' will almost certainly be proven.