Actually if the fireworks are at 9:00, which they were last Dec while we were there, the EWP begins immediately(within seconds) of the ending of the fireworks.
As for the times, I got this from allears.com....
"Another treat that comes with staying at a Magic Kingdom resort that's not as publicized as it should be is the Electrical Water Pageant. This convoy of beautifully lit floats makes a musical circuit around the Seven Seas Lagoon nightly, weather permitting, passing in front of the Polynesian around 9 p.m., the Grand Floridian at 9:15 p.m., the Wilderness Lodge at 9:35 p.m., Fort Wilderness at 9:45 p.m. and ending at the Contemporary around 10:05 p.m. Good viewing spots can be found along the beaches of any of these resorts, as well as from select restaurant locations -- Narcoossee's at the Grand Floridian, for example, affords an on-the-waterfront seat. Try to see it if you can"