One of the reasons why Windy cannot provide ECMWF GRIB files is because of licence agreements:
Then, as you say, it should be necessary to use a special Windy GRIB application and sailors would ask too for an additional routing function.
For shorter passages, a good improvement on Windy should be to offer the possibility to keep in cache the forecast up to 5 days for a limited region. But this is not suitable for longer passages.
(To do that for 2 or 3 days offshore, I use the video screen capture on iPad, but then you need to stop regularly and have a beer in a WiFi spot bar to refresh the forecast... and you too)