You could try this method but (and isn't there always one)
you have to take into account any currents that may be running and the fetch (distance wind has to travel over the water where you happen to be bobbing up and down). There is also the wind shadow from hills the difference in local conditions within a few miles of each other. etc etc. Having a wind guage on hand would be the only way you could accurately report the wind strength in your area but I don't think it's something I would be carrying in my pfd. It may give you bragging rights at the pub but........