I love both the Disney water parks, really I do. But even for a visitor who only gets across every couple of year.....things have gone stale, very stale.
It's now 18 years since I first went to either.....and if I'm not mistaken only Crush 'n Gusher has been added. Even that is a quite disappointing, long waits, carrying tubes for a mildly exciting ride.
Thinking mainly about domestic visitors....the issue I see, even if the new waterpark is by far the best in the world, it's still not going to move the crowds from Disney.
For many, the addon days at a Disney waterpark are a very cheap or low cost addition.
Where it works better is trying to get people to take a Universal only break, a waterpark will likely help them decide to stay on-site and not visit Disney because there's a good 5-6 days worth to see at Universal.
For international visitors who typically come for 2 weeks, it's a bit different because you've always been able to fit both Universal and Disney (although Disney's sneeky ticketing tries to persuade you not too).
I really hope Universal produce a really top class park here. I'm still probably going to prefer the theming of the Disney waterparks (all comes down to the character sets), but will sure look forward to Universal's new offering. I would love to think this will force Disney to up their game, but for the reasons above I'm sceptical.