The flower is pollinated by scarab beetles. As can be seen in the first figure, the pistil of the water-lily lies at the base of a hollow chamber like structure surrounded by the stamens. This hollow chamber hides the reward for the beetles in form of sugar and starches. The flower bud develops under water and when ready to open, it emerges above the water surface between the surrounding leaf pads during day time. However, the buds open only as the sun sets and the white petaled, fragrant flowers draw the beetles. The beetles have to crawl into the inner chamber to get their bounty. As midnight approaches, the flowers start closing in, thereby trapping the insects feasting on them. The closed petals start changing their color from white to pink.
To witness this saga of sweet entrapment, watch a very informative and beautifully shot, short documentary by David Attenborough.