Waterlilies will be fine in the same pot for a while.  It may be time to repot when most of the rhizome has “jumped” out of the pot.  Repot in the same manner as described with the marginals, making sure to leave the crown above the soil line.  Removal of small offshoots will help the main crown produce more flowers. 

Water lilies and lotus need at least 6 hours of sun to bloom their best, although there are a few exceptions, such as hardy lily ‘Chromatella’ and the blue tropical water lily.  Pots should be placed so that the crowns are 8-24” below the surface, although a few larger varieties can grow in water to 36” deep.  Waterlilies and lotus resent splashing water on the leaves, so avoid placing near fountains or waterfalls.  Make sure that at no point in the summer oxygenators or surface floating plants block the light reaching the crown, as this will reduce flowering.  Each leaf on a waterlily lasts only about one month during the summer, so removal of yellowing leaves down to the crown may keep them looking their best.  Old flower stems may be removed in the same manner, thus helping both lilies and lotus to rebloom.