Or have ridden, for the Surfdopers out there. 
Me it was just after the hurricane we had go past here about 5 years ago (2002 I think it was). I went out to the beach after the all clear, and they must have been at least 20, maybe 25 feet (this from trough to crest), making a horrendous boom when they broke. Some guys tried to paddle out there, but, despite the offshore winds, the side current was roaring down the beach and the waves so powerful I mostly saw exhausted surfers staggering out of the surf, never having caught anything.
I saw 15-18 foot swells on my sailing trip. They topped out over 20, but I wasn’t on deck at the time.
I once saw a wave in Fenway go over two dozen times, including through the new monster seats!
10 foot waves are intimidating, much more than that and you’re gambling with your life. Anyone who’s been under more than a couple minutes can verify that.
15 years ago off the coast of Norway. The oil rig I was working on had shut down due to somewhat inclement weather. A few hours later, a wave (or part of one skimmed off by the wind) hit the rig floor. The rig floor was 130 ft above sea level. it wasn’t a fun few days.
IN, IIRC, late 1975 I was stationed on the USS Canopus in Holy Loch Scotland. We sailed for Charleston in late November or early December and hit an enormous storm that followed us for about ten days. During the storm, we had winds of over 113 knots and the waves were incredible.
Sailors were ordered to stay inside the ship for fear that they would be blown overboard and there was no way in the world they’d be able to pick you up. One morning, in complete defiance of orders, I went outside. I was standing on the first weather deck and looked up at a wave. No way for me to estimate how high the wave was, but you can take a look here to see the size of the Canopus. http://upload.wikimedia.org/wikipedia/en/8/8f/USS_Canopus.jpg
Regards
Testy
I can’t really say how high they were, but one time when I was on the ferry towards the place I now live (Isle of Man) I saw waves so huge that they were too big to act in the normal way on a large ferry. The ferry just sat one side or another of huge waves. I’d say these waves were about the same size as the ferry.
I love a rough crossing. A smooth one is mucho-boring.
That’d be the Indian Ocean tsunami on December 26, 2004.
After that, some really gnarly stuff off Half Moon Bay after a storm.
Just like this, in the North Atlantic. Waves were at least 50 feet. It was on an LST (Landing Ship, Tank) (FLAT BOTTOMED SHIP). We took a 53 degree roll one night. Not pleasant, I can tell you.