There really is no way to know about the fireworks. Figure that the parks will be closing fairly early, most likely 6:00, and the fireworks would probably be at that time. On a Wednesday, I wouldn't think it would be open later than 7:00, so if you want to be safe you should plan your dinner after that.
The water pageant is much later and always runs at the same time. I know it passes Wilderness Lodge around 9:30, then goes on to Fort Wilderness and back to MK. Honestly, though, that one isn't worth planning your dinners around. Catch it if you can, but if you miss you won't have really lost out on much.