It seems there are many methods of overwintering Tropical Water Lilies. According to popular opinion, each method works about 50% of the time. Fertilization should cease when your water temperature starts dropping in early fall. This will promote the growth of small corms to store energy for the winter. These small corms (look like walnuts) are joined to the mother plant and can be successfully broken off to produce new plants for the next season. Then wait until the water gets to 55-60F.

If you choose to wash and store the Tropical, remove the water lily from the pond and remove all of the soil around the plant, washing it all away to expose the root mass and corms. If small corms are present, remove these and place in very slightly damp peat moss or sand in a paper bag. The moisture should not be great enough to stain the bag. Place only corms from one lily in the container and label it so that you know which type of plant it is. We have better luck with paper than plastic. It is recommended that you air dry and dust them with a fungicide before packing them. The fungicide will help to prevent mold while they are in storage. A greater number of plants seem to get moldy in sealed containers. Check the plants every month to assure that they do not dry out completely (brittle).
Store them in a cool dark place with temperatures between 50-55 degrees.
If the corm gets mushy, discard it, saving only the hard ones.

For the mother plant, remove all leaves and trim away excess roots. Place the mother plant, dusted with a fungicide in some damp sand also and keep in an area of the house that will stay above 60F. If your winters are short, you might consider keeping the mother plant alive in an aquarium. The corms can be added a month before your pond water will reach 60F degrees.

If you have the space, you can bring in the entire plant for storage. If you chose to do this, trim off the largest and any damaged leaves so the pot fits into an aquarium or pond inside.
Keep the temperature above 60F to keep the plant alive. Let the water slowly evaporate over the winter and add water in the early spring when you are ready to start growing the plant again. Some supplemental lighting may be required to give the plant a bit more energy and keep the plant alive.

In the spring, once the water temperature is safely maintained above 60F, pot the stored plants in fresh soil with fertilizer tabs and place in the pond 2-4” below the water surface to get them started. When new growth appears, start moving them lower toward their appropriate place in your pond. Once any new tender green growth is seen, be especially careful to not let them experience water below 60F degrees. Young plants can fall back into dormancy and may not have the needed energy to re-grow. Keep an eye on your Koi. Their browsing can rapidly kill young plants.

A single corm or tuber can produce 20 baby plants. This takes some care in handling. If you will be propagating these individually, don't pinch off the new plants too early. It can be best to let them grow as a mass, and then carefully separate and pinch them off when they are getting too large for their space. Plants propagated in sand are much easier to separate this way. We use ten gallon aquariums with heavy lighting to start sprouts.
Mass plantings are best grown on a pond liner with 3 inches of water depth above 3 inches of sand in direct sun. The dark color absorbs plenty of early season heat allowing the plants to grow rapidly. Remember to mark the names of the plants. We use thin white plastic cut into strips and written on, then attached to the rockwool growth blocks. Fertilize the water rather than the plants and use a small pump to circulate and keep some movement in the water. If the water gets above 80F, shade the area as needed. One hot day could heat and evaporate enough water to cook your young plants. When our air temperature stays in the 70-80 range, we transplant and roll up the liner.