Hi landroos,
The default wave height works on the Beaufort scale which is a 0-12 range. However you can set the waves to custom and increase the large wave height to be higher.
If you need even larger waves you should be able to edit the UI editor to allow larger numbers. Try opening the Editor/suimono_object_editor.cs file and around line 477 you'll see the following:
EditorGUI.LabelField(new Rect(rt.x+margin+10, rt.y+200, 120, 18),"Large Wave Height");
script.lgWaveHeight = EditorGUI.Slider(new Rect(rt.x+margin+165, rt.y+200, setWidth, 18),"",script.lgWaveHeight,0.0f,4.0f);
Changing the '4.0' at the end of the second line will allow you to make the max wave height larger.
Hope this helps!