@Matthewdupreez - is that an experienced radio propagation engineers response, based on experience, or a guess? (It sounds like a guess given how 5.8GHz and water interact with each other)
Water does interact with 5.8GHz quite strongly and very well could have an effect (and its interaction is going to be different the the ground). A quick bit of googling shows that for
point to point links over water has a recommendation of the antenna being at least 12m off the ground to minimize the effect of the water.
The water can reflect the signal and cause multi-path interference (that link above shows some effect), if your using CP antennas, this impact should be reduced as good CP antennas tend to cancel out/reduce multi-path interference.
I am not sure how much of an impact it will create as I have seen people do flights over water a good distance, even at only 10m off the ground.
What type of flying and distances are you worried about
@sumone , that might give us some context to better answer your question?