So 19' would be 9.5' +/- flat water. How the weather people actually measure this I have no idea, my guess is from ship weather reports, when I was in the NAV we used to radio out a weather and sea state report every watch (4 hours), I guess they were recorded and collated somewhere...

20' is considered a heavy sea, but keep in mind the crests are quite far apart, so it is not as terrifying as you might think. The steep seas you see on surfing beaches do not occur in deep water offshore. It is very difficult to judge their size and shape, particularly from a small boat, and sailors notoriously exaggerate the heights of the seas. The largest actual sea ever measured (in a hurricane)was 112' in 1933. The height was triangulated from a ship in the trough where the bow was at the crest. But it was a big ship (500')and the vessel was not in danger.