The problem Universal is having now, is that they are running at 90% occupancy rate at the resorts. Since they can't really get many more people in their hotels, I predict there will be less people in the parks, since a big batch of those staying on property will be at Volcano Bay. But the major variable is how many people will come to UOR for Volcano Bay and stay off property.
What Volcano Bay will do is make getting a hotel room more difficult. Since the people that are staying on property will likely extend their stay by a day.
But to answer your question? It's June and they are opening a brand new water park. It will be busy.