The Swell2 arrow does not always show zero.
Here it shows 0.4m:
Are you talking about the slope angle of swell?
Breaking waves are observed near the seashore where the water depths are reduced and when the ratio height/wave length is about 1/7 i.e. the wave steepness dramatically increases.
Global waves models like ECMWF WAM and WW3 are not able to show that because their resolution is too low (13km and 22km) in particular to take into account a precise topography of seabed.
More info on surf breaks: