Tropical Water Lilies can be replaced each year or overwintered indoors. If you choose to over-winter them there are 2 methods.
Keep them actively growing by placing them in warm water with bright light and warmth. It will be difficult to maintain a minimum water temperature of 21°C (70°F). This may only be possible in a greenhouse or sunroom. . If your Lily survives it can be returned to the pond when the water temperature reaches 21°C (70°F). This is usually in early June.
Over-winter a tropical Lily by letting the tuber go dormant. Leave the plant in the pond until after a killing frost. The cold water will induce the formation of a small, black, grape-size tuber. Older specimens may have already produced several. Gently rinse all the soil from the tubers and float them in lukewarm water for a day. Viable tubers will sink and spoiled ones will float. Discard the spoiled ones. Place the viable tubers in damp peat moss or sand. Store in a cool basement. Check them from time to time to make sure the peat moss or sand doesn’t dry out completely. In early May check to see if the tubers have any sprouts. If there aren’t, place the tubers in distilled water in a sunny window to break dormancy. When roots are 1-2.5 cm (1/2” -1”) long pot them 6 mm (1/4”) deep in heavy soil and submerge the pot(s) in 7 cm (3”) of water. Maintain a minimum water temperature indoors of 21°C (70°F). When leaves emerge and the pond is at least 21°C (70°F) they can be safely returned outdoors to the water garden.