I've done the measurements many a time on Google Earth. Aquatica's, Typhoon Lagoon's, and Blizzard Beach's perimeters could fit into the plot between CBBR and RP with a different layout. Aquatica has a perimeter of a little over 3,700ft, Typhoon is about the same, Blizzard Beach is a little over 4,000, and the usable area of the plot between the two hotels has almost 4,200ft. Now, the layouts of all 4 parks would not fit there, because they are circular and the Universal plot would require a rectangular shape, but a full sized water park is possible there.
It's all up to how they do the layout and how many guests they're looking to attract. Also, parking would probably come into question, as the parking structures may begin to get pushed to their limit after Potter 2.0. But they could maybe put the water park's parking lot on the remaining land next to CBBR.