My question is this, the construction of the floats for the Cessna 206, when the aircraft is sitting in the water the weight is carried not just by the very bottom of the float, the köl (I suppose you call it keel) but by as much of the float as is submerged, is this correct?
That is essentially correct. The numbers in the rating of float indicates its buoyancy, or how much weight it will support. Flotation rules are contained in FAR Part 23.751 and states:
(a)...Each main float must have...
(1)...A buoyancy of 80% in excess of the buoyancy required by that float to support its portion of the maximum weight of the seaplane or amphibian in fresh water; and
(2)...Enough watertight compartments to provide reasonable assurance that the seaplane or amphibian will stay afloat without capsizing if any two compartments of any main float are flooded.
(b)...Each main float must contain at least four watertight compartments approximately equal in volume.
That 185 in your video is equipped with Edo 2960 floats, and its MTOW if I recall correctly is 3100 lbs, thus it is very well floated with nearly a 100% buoyancy reserve. Other aircraft aren't so well off, the 8000 lb MTOW Otter is underfloated on the standard Edo 7170's. My opinion, of course.
Float keels are made from aluminum, but usually have a steel shoe. Landing on wet grass like the video will not damage the keel, maybe rub a bit off the screw heads. Water is a pretty hard medium at the touchdown speed of the average seaplane. The risky thing about grass landing with a seaplane is if you do it often enough the genie of bad luck will pee on your careful planning and you could end up inverted. The guy in the video deserves an attaboy. He does everything exactly right, particularly the attitude during the approach and touchdown. Any flatter and he'd be touching down too far forward of the step inviting a nose over, any more nose up and the heels of the floats will touch first, forcing the airplane forward. Nose over time!
You can land on ice on floats but with a caveat. The ice should be thick enough to support the weight of the airplane. If you break through, ice can hole a float very easily and you lose the airplane. A company I used to work for did exactly that with a Beaver many years ago. Personally I wouldn't land on ice on floats unless it was an emergency situation.
Here's a vid of a float takeoff with a loaded Norseman under no-wind, glassy water conditions. Check the length of the takeoff run, it's over a minute and a half. You'll see he doesn't try to force it onto the step, simply allows the airplane to rise, then when the spray from the float is about even with the wing struts he begins to ease on forward elevator to bring the airplane up onto the step. Then at the end he raised the right wing, lifting the right float out of the water to minimize drag, and at last he's away. Classic Norseman takeoff.