Offshore you can install bigger turbines, because sea transportation of turbine components is not size-limited like rail/road transport. Taller turbines help to reach wind resources that are stronger and steadier. Also, even at the same height, offshore wind resources are often better than they are a few km inland. Finally, offshore wind can supply power to densely populated coastal areas when there isn't room to build turbines on land.
The disadvantages are higher construction and maintenance costs; waves and salt water are much more challenging to materials than the ordinary conditions onshore turbine towers experience.
The turbines are anchored to the sea bed. The wind has a boundary condition against the ground. If the ground is the sea, then the sea current wrt to the sea bed adds to the wind speed wrt to the sea bed.
Therefore, the turbines have zero wind speed only if the wind is blowing exactly against the sea current. Which is hard and rare.
Because the land boundary condition is different, the further away offshore you are, the better the effect is. Also, you might as well pick spots with known stable sea currents.
The disadvantages are higher construction and maintenance costs; waves and salt water are much more challenging to materials than the ordinary conditions onshore turbine towers experience.