Wind is an underused renewable resource, and the ocean is nearly always windy. Open ocean has advantages for wind turbines. It's flat: no physical obstacles to wind, such as hills and trees. For all the advantages the ocean offers for wind turbines, there are some significant obstacles as well.
The ocean generates more wind than land does. In fact, waves are proof of the existence of wind. Waves are created by wind. Land-mounted turbines may have to rise above obstacles or deal with complex geography that creates inconsistent winds; not so with ocean turbines.
Population Centers and Noise
One of the biggest complaints about wind turbines is their noise. If they are located in the ocean, away from population centers, there aren't neighbors to disturb. An entire farm of turbines could run without disturbing anyone. They may, though, be visible to population centers, a marked disadvantage.
Construction in marine environments is extremely expensive, even more expensive than remote land locations. It requires fastening a tower to the base of the ocean. According to Green Living Answers, depths are currently limited to "about 60 to 90 feet." While this is less difficult than at some of the ocean's extreme depths, it is still extremely expensive.
Maintenance Cost and Power Distribution
The cost to maintain any ocean equipment is relatively high. Salt water is corrosive. The effects and expense can be seen between the difference in maintenance costs of fresh water vs. saltwater boats. In addition to the harsh marine environment, workers have to travel to the locations on boats, which adds another layer of expense. The infrastructure -- the power lines -- need to run from the off shore generators to land. This presents yet another high cost relative to land-based systems. However, unlike real estate, the ocean surface doesn't have other, competing commercial utility. So, land acquisition costs may be saved.