The 500mm II I can hand hold for short periods, still a monopod is prefered.
I could hand hold the version I for short bursts, pull it up hit and release. Long holds were difficult.
With the 600mm II has about the same weight as the 500mm I and the length is about the same as with the extender.
The added length makes it even more difficult as the center of balance is farther away from your body.

I wouldn't buy the 600mm thinking I was going to hand hold it very often.

In Johnathan's picture if I didn't have my monopod I would use it just as he is, but I would be looking for a way to brace against the boat or looking for a stool to sit on for maximum support. Also how you posture makes it easier to hold to. I hold my elbow in close to the body to provide better support. I think of it like shooting a firearm, the more stabilized your elbow is the better the results.