Excuse my ignorance but why would you want to try to calculate it...?
If the wind is blowing check the water for the wind direction, and land accordingly. If there's not enough wind to pick up a wave then it is neglegible.
Trying flying parrallel next to the boat, keeping same speed and heading and check what you ASI gives you as this is what you'll be landing in.
If you're dead bent on getting results, I suggest using this
APR E6-B2WHL Pilot Flight Computer - Marv Golden Pilot Supplies
I bought one for my JAA exams and the wind arm worked a treat and is quick and accurate... and you won't have to take your hands off the controls to fuddle with pens and rulers
