From the official 1964/65 guidebook:

In a show sponsored by the Florida Citrus Commisision, teams of performers water-ski behind fast boats, doing intricate acrobatics, formations and jumps, four times every day between noon and 6 p.m. A highlight of the Seattle Century 21 Exposition, the show is staged on a specially constructed, doughnut-shaped pool in the Amphitheatrer first used at the 1939/1940 Fair.

I liked the description of the show from the 1939 Fair better:

In the Marine Amphitheatre on Fountain Lake, famous Broadway Billy Rose stages his "Million Dollar" Aquacade, starring Eleanor Holm in a brilliant "girl" show of spectacular size and content. Here you witness one of the most enchanting panoramas of all time. The dancing ballet goes through its routines, the water ballet weaves nymph-like figures in the crystal depths, Olympic diving champions flash through the air, and Aquabelle Holm exhibits her allure to the accompaniment of orchestral music harmonized with multi-colored lights.

Seating 10,000 people, the Amphitheatre looks out over the water toward a stage 200 feet deep and 311 feet wide, while between the two a water-curtain aglow with color forms a spectacular, filmy barrier. Eight thousand gallons of water a minute pour into the making of a man-made Niagra which stretches 260 feet and rises 40 feet in height. Four pumps operate to keep it suspended in space. Two hundred and fifty 2,000-watt lamps and sixty-four 4,000-watt lamps are placed at the front of the roof over the Amphitheatre. On each side of the stage, ninety-six 2,000-watt lamps are used for side lighting effects, while eight 150 ampere arc lights provide the sources of color.