Now that's a little different. I don't think Disney is worried yet but if they keep losing market share to universal that's when the worry starts. They lost I think .9% in market share this past year. While universal gained 1% or so. The new water park isn't going to change things a whole lot except maybe people go to volcano bay instead of one of the Disney water parks. People aren't going to skip one of the theme parks for a day at volcano though.
Once universal announces that third gate, that's when things will become serious.
In the info UO sent out to AP holders they announced Volcano Bay as the 3rd gate. They are calling it a "Water Theme Park like no other".
I think Volcano Bay may have a slightly bigger impact than you are anticipating. They are building a 6th onsite resort across the street which will mean 3 resorts that are basically attached to VB and shuttle rides to the entrance from the Deluxe resorts.