I am not specialist in offshore wind energy. However, there are at least three factors to take into account: high installation costs, complicated logistic and maintenance and technical issues of the power transmission.

In addition to above mentioned issues, weight of the unit itself will pose a greater challenge. Since, lighter turbine generates less torque leading to lower power generation.

just adding to what the other have already said. you need to consider the platform design, mooring and tether - in detail. The climate in the region will heavily influence these designs as they need to face (and survive) storms and cyclones. You may want to look at how your platform (and tower) moves in response to extreme sea states, dynamic and static loads on mooring and tether. I am not sure if the motion of the turbine platform will have a significant impact on energy extraction but it is worth looking into. Take care to look into overtopping moment as motion is less restricted in floating turbines and wind speeds can be high in open seas - again this adds to your dynamic loads. Eventually the design of the airfoil and blade are also affected by these factors. You may have to chose survivability and  over efficiency in such environments. There are many factors at play which span across disciplines from design to installation stage. 

https://www.wind-energy-the-facts.org/factors-affecting-turbine-location.html

http://www.eolmed.fr/en/the-advantages-of-floating-wind-power/