no, it's actually not that bad on the knees at all. in all of the years I've skied (comp), I think I can count on one hand the number of blown knees I've seen. if anything, it's rough on the shins when you start doing tricks (and crashing).
invest in some HO wake skis - they make the best ones I've seen. it's not like walking off of the mountain and on to the water...it's a little different, but should be a fairly easy transition. it's a lot more of an upper back workout than snow skiing as you're being pulled as opposed to being pushed.
shit, I meant to leave you this. these are the skis you want - size should be right. I don't know what size you are, but unless you're 8 years old and weigh 75 lbs, the 130s are going to be way too damn small. (and even if you are 8 you won't be 75 lbs for long - get the bigger ones.)
you can find the ski binding combos for cheaper than they're listed here, but this will give you an idea of what to look for.
i don't think so. the mounts for all of the wb bindings Ive ever seen are too wide for the ws.
i got pair of these bad boys last summer this is what i haveto say for them
they are more easy to use than water skiis, they are lighter so you could potentialy get more air.
they have wakeboard bindings incase you want to do spins or flips
fins on both ends so you could go switch
i tried to do a 360 off the wake and landed but couldnt keep going
they are pretty fun if you know how to waterski well but unless you do, its going to be almost the same as regular water skiing
hope this helps