In my opinion, waterskiing will always be alive and well. Everything is cyclical, just look at the rise and 'fall' of wake boarding. For a few years that was all you saw. There are still a lot of people that do it, but its not nearly as prevalent as it used to be. Water skiing may be the next big thing in water sports once surfing starts to die down, who knows. Just look at the saying "Whats old is new again!"
I think in terms of difficulty it goes (from hardest to easiest) Slalom, wakeboard, surf. This is why we saw the rise of wake boarding 10-15 years ago and now surfing in the last few years.
I think getting people out on the water and experiencing behind a boat doing whatever activity they want is the most important thing. I still have memories of first getting up on skis, first time cutting hard on a slalom, first time jumping the wake on a board, first time surfing rope-less. People will gradually work their way up in terms of difficulty and I would not be surprised if a lot of these people surfing right now find themselves on a slalom ski in a few years time!