Actually, I beg to disagree in that I believe true glass in daytime to be worse (that is until its time to try to get out). The usual illusion is that you are higher than you actually are, however I've heard of seaplane pilots trying to land 50 feet up.
Also, with glass there will be no wind component to use to reduce impact speed and there can still be large swells which are impossible to assess. Glassy water is also a lot stickier and you will find the horizon just disappears as you approach the surface.