Just to clear up any misunderstanding on my part, for most multi-day tickets above 4 days, you aren't really paying "more" to park hop or to visit the water parks compared to the previous Park Hopper tickets under the old program that offered similar options. In fact, for 6 or 7 days, the passes are cheaper than the old, but I digress. While true they are now listed as an "add-on", I don't know if its technically right when its said "now they are making me pay to Park hop" when taken out of context. Rather, what would probably be more appropriate to say is now you don't have to pay to park hop or visit the water parks if you never planned on doing it in the first place, which were automatically added under the old program.
That point is kind of mute as with time (in fact not too long), it will be seen as if they are making you pay to park hop and visit the water parks, which is also technically true. This may or may not have an effect on attendance. Where it will come in to play is for those folks who never planned on going to water park in the first place. Under the old program, that option was automatically tagged onto your ticket. This might have created more incentive to visit as you've already paid for it, so why not? During the summer, the water parks in the past can get insanely busy. A few less bodies won't hurt too much, imo. But yes, I can see where it might be possible that water park and other "Plus" locations could see attendance drop with this new pass program....probably not that significant though.