This is why it is important to select the right forecast site (which uses a certain model) to match your needs.
For oceans synoptic scale models are used (such as GFS) and provide good results, but use the same models for the Med and you get unreliable forecasts - they will provide the "big picture" but this is useless for you being say between Kos and Astipalea.
University of Athens (
http://forecast.uoa.gr/gmap/) and Openskiron.org use different models (SKIRON and WRF respectively) which are on the mesoscale level and consider local effects due to islands, mountains etc. - It is more difficult to provide good forecast at this level but these sites do a pretty good job at the 2 days high resolution, and the 5 days low resolution, which is typically enough for the Med.
I usually consult one site that gives the "big picture" like passageweather.com and compare the results from these two sites - if they show the same thing then great, if not then I compare which of them is closer to the big picture (and then again sometime it is the other one that is correct).
For those who do not use OpenCPN or do not have a PC onboargd (which can run ZyGrib) a useful application for mobiles is SailGrib