It's a complete mess trying to get accurate marine forecasts. I pay most attention to wind speeds/direction and use that as my guide. Anything over 15mph and you figure on having 3ft waves or higher. High teens and you are in 3-5's, with rogue waves up to 6ft. When you get 20 mph sustained then expect a bumpy ride with 4-6 ft?
Here is a site to go by

http://www.unc.edu/~rowlett/units/scales/beaufort.html