The big difference is that your knees are side by side when snow skiing. So rolling them one way or the other is no issue. When your on a slalom ski with your feet in line, I can't see how you could do that without sending your upper body mass outside the turning circle.

The only time you initiate a carving turn in snow skiing with the knees only is at very slow speeds or for a very short-radius and short-duration turn. Initiating with knees only puts you in a weak and vulnerable position that is short-lived. Look at World Cup slalom turns (a short-radius and relatively short-duration turn) - most of the angulation is coming from the hip and lower torso. At higher speeds you need to move your mass inside the skis (and forward preferably) or you will not be able to withstand the centrifigul force and will "high side" your turn. Given that in slalom water skiing all of our turns are relatively high speed, you wouldn't initiate a turn with your knees only. Besides, at what point other than in a traditional gate entry are you on a flat ski where you could actually initiate a turn with your knees only? As soon as your ski changes edges you have essentially initiated a new turn. If you are referring to the finish of a turn as in coming back to the handle from the turn's apex, then you already have your mass traveling on a line inside your ski and you don't need to initiate anything. At this point you might add to the edge angle of your ski with your hips or knees to decrease the radius of the turn (is this what you are referring to?). I have played around with this some. When approaching the bouy your mass is already inside your ski, your ski is already on edge and you are already already turning, so add some edge angle if you want, but for the most part, ride the ski through the turn.
Jim

When thinking about the finish of a turn, remember that any pull that you make is going to be against a 3000 lb. boat moving down course at whatever speed you ski at.  Nobody is strong enough to overcome the boat, all you can do is use it for support by maintaining a tight line through the turn and then falling back into the handle without trying to take anything from the boat other than support for your own bodyweight. By trusting all of your balance to the rope, you free up your ski to carve the turn that it wants to resulting in the correct amount of angle back into the wakes after an arcing/ continuous turn. Don't know if this makes sense but I learned it from those better than me. Wade is an excellent coach and I hope you get the chance to ski with him.

so smooth...

The key is to see what you want to do before trying to do it.  That is the reason that we made the film Slash.  Watch the skiers in there and watch segments in slow motion to see what's happening.