Nymphaea are grouped by scientific classification in the larger category, order Araceae
You know other "arum" members; these were some of the first terrestrial flowering plants, invading the land back in the Devonian Period (some 180 million years ago)
The stately magnolias are also arums; look at their and the water lily's large flowers. Both share ancient characteristic "redundancy" in their parts; multiple petal, sepals, and each is 'perfect', bearing both pistils and stamens.