I'm not a pro boater, but i have had a couple scarry rapids over the years. Two that stand out are Big Brother at just over 4 ft. (here's a pic of the weekend before...it was a good half foot higher the next weekend)
the second was dropping into Energizer rapid, 100 foot tall flume slide, on the Toxaway River, NC. with just verbal directions, no scout. rolling over the lip was terrifying.