Someone was asking me about my class IV comfort level the other day, and my answer was something like "I'm comfortable that I can usually find an eddy to swim to". This inspired me to write, 'Dave's International Scale of River Difficulty':

Class I: Easy. 
Fast moving water with riffles and small waves. Swimming is pleasant, shore easily reached. Almost all gear and equipment is recovered. Boat is just slightly scratched.

Class II: Novice. 
Straightforward rapids with wide, clear channels which are evident without scouting. Swimming to eddies may require moderate effort. Climbing out of river may involve slippery rocks and shrub-induced lacerations. Paddles travel a great distance downstream requiring a lengthy walk. Canoe hits a submerged rock leaving a ding on gunwale.

Class III: 
Intermediate. Rapids with moderate, irregular waves which may be difficult to avoid. Water is swallowed. Legs are repeatedly ground against sharp pointy rocks. Several eddies are missed while swimming. Difficult decision whether or not to stay with boat is made. Decision to stay with boat results in moment of terror when swimmer realizes they are downstream of boat. Paddle is recirculated gently in small hole way upstream. All personal possessions are removed from boat and floated in different directions. Paddling partners run along river bank shouting helpful instructions. Boat is munched against large boulder hard enough to leave series of deep gouges. Sunglasses fall off.

Class IV: 
Advanced. Intense, powerful but predictable rapids requiring precise swimming in turbulent water. Swimming may require 'must' moves above dangerous hazards. 'Must' moves are downgraded to 'strongly recommended' after they are missed. Sensation of disbelief is experienced while about to swim large drops. Frantic swimming towards shore is alternated with frantic swimming away from shore to avoid strainers. Rocks are clung to with death grip. Paddle is completely forgotten. One shoe is removed. Hydraulic pressure removes car keys and credit cards from zippered paddle jacket pocket. Paddle partners running along stream look genuinely concerned while lofting throw ropes 20 feet behind swimmer. Paddle partners stare slack-jawed and point in amazement at canoe which is finally stopped by major feature. Climbing up river bank involves an inverted tree. One of those little spring-loaded pins that attaches watch to watchband is missing. Contact lenses are moved to rear of eyeballs.

Class V and beyond: 
Expert. Not recommended for swimming.