Answer:  

Dr. Shoshiro Minobe kindly answered the question above.
 
 
In theory, I think that water could freeze at more than 0 C°.
  
There are two effects, sensible heat and latent heat, which take the heat out of water. Sensible heat is the effect by direct heat exchange, and latent heat is the effect by evaporation. 

  
In regards to sensible heat,
  
Heat flux taken from water = Constant × Wind velocity × (Water temperature - Air temperature)
  
This indicates that water will not lose heat but gain heat, if the air temperature is high. So 0 C° water is warmed up when the air temperature is more than 0 C°.
  
However, as for latent heat, 
  
Heat flux taken from water = Constant × Wind velocity × (Saturation specific humidity - Specific humidity of air)
  
So air takes heat out of water no matter what the air temperature is.
  
Therefore, if air is very dry and the air temperature is only a little bit higher than 0 C°, water will possibly freeze because the effect of latent heat is more than that of sensible heat in this case. However, I don't know whether this really occurs in nature.
  
  
Acknowledgement
Dr. Shoshiro Minobe
He is studying about the climate change related to air and ocean.
  
 

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