Is windguru reading normally fairly accurate or is there a bit more wind once on the water?

Way more wind on the water, don't know what they are reading. Yesterday was starting to be overpowered on 8m lots of people on 6's but windguru was at 15. If its saying 15-16 then it's usually a great 8m day.

I use the Kahululi airport sensor for kite beach:
http://www.ikitesurf.com/windandwhere.iws?regionID=1700

Also, some friends and I went to Maui back in August and the forecast was completely off the whole time. The forecast would say 14-18mph and it'd blow 25-35mph from from noon-sundown. We were on 7s, 9s and 134cm boards the whole time. It was awesome!!

I went around mid august as well, was out on an 8m and surfboard, I'm 230lbs, forecasts were light. After a while I notice I'm 1 of about 3 kiters out, started getting worried they knew something I didn't, then I realized everyone went back in to switch to 5 and 6m kites, haha. Maui is definitely another level

WindGuru is great. It is pretty accurate as far as the ups and downs throughout the week but the numbers are always under the actual wind speeds. You normally have to add 5 to 10 especially if the wind direction is out of the east. I tell people to bring your full quiver to make sure you are covered. Basically everyone rides a 7 or 8 everyday but sometimes you would like to have that 5 or sometimes a 12 or 14.

Try this site: http://weather.mhpcc.edu/wrf/maui2/
This is probably the most accurate site I have found. It gives you a map of Maui County showing wind speeds and direction on an hour by hour basis for the next 2 days.

Aloha