It happens every night, unless something has happened.
Although the pageant generally follows the schedule below, check with Guest Relations or Lobby Concierge at your resort for times during your stay. Inclement weather can cause cancellation of the pageant.
Polynesian Resort -- 9 p.m.
Grand Floridian -- 9:15 p.m.
Wilderness Lodge -- 9:35 p.m.
Fort Wilderness -- 9:45 p.m.
Contemporary Resort -- 10:05 p.m.
Magic Kingdom -- 10:20 p.m. (only during extended MK park
hours)
Note that when the Magic Kingdom Fireworks are scheduled for 9 p.m., the Electrical Water Pageant runs about 7-20 minutes later.
You can see it from the beaches and docks at each resort. I would personally advise getting to the resort you've chosen to watch from about half an hour prior to the time it should go by so you can get a seat or a good standing spot.