I have had the same problem with drag and spray while pulling an inflatable and took the advice of other posters and purchased the booster ball. It will get a workout on Labor Day. I'll let you know how it does.
You say your using the "big thick rope", what is the size of the tube?
Those large diameter ropes are made for very large tubes. Like the Big Mable that are large and heavy.
If you have a one or two person tube, or in general, a light weight (I don't mean cheap, I mean the actual weight of the tube) tube. The rope is too big and when it gets wet, is too heavy. That is most likely what creates the spray and drag.
You need a smaller diameter rope that does not "soak up" water. My "big" rope is a softer, loosely braided rope that actually "holds" water making it even heavier. The smaller "tightly" braided ropes, I believe nylon, are much better for the smaller lighter weight tubes. Go to the Wallyworld and pick one up and give it a whirl.
Thats ridiculous. If the rope is taut it won't drag. Sounds like he connected it incorrectly to the tube.
This all makes sense, however, as I mentioned, sometimes the tow hook is just too low. Like I said, we have used both boats with the exact same tube and rope, and on mine it is fine, on my buddy's it drags in the water. His hook is lower than mine. Both boats are a little over 21ft, both V8s, both bowriders. Main differance is the height of the towhook.
Aside from putting a wake board tower on there or some other radical solution, what can my buddy do?