@ivo Hi I'm sure you've heard from lots of surfers since this post went up, but I got here because I was looking for more options on viewing swell in Windy. I use Windy for general weather/wind, but so far cannot use it for waves because it does not offer some visualizations that are important for us. @jrbidlot 's post below is a great explanation of the various parameters that are important for ocean waves. Since I don't know ECMWF website as well though, I'll link to some pages from NOAA Wavewatch.
For surfers, we rarely analyze one single parameter. They all are important. But two that are missing from Windy are the option to show primary and secondary swell periods (vs. showing mean period). As you can see below, this shows the timing of different swell events nicely, and swell period is important too since energy scales so much with period in shallow water. Different swell periods/directions will "work" for different breaks.
Also, note the arrows for direction, these are also quite useful for significant wave height display.
Thanks again for all your work on Windy, it's very useful for local wind prediction, which is also super important to surfers!