When fall and winter rolls around, then it is time to start caring for the pond plants and their winter sleep.
In mild climates such as zone 8 and up, little care is needed for plants such as grasses, reeds, rush, and hardy water lily. 

In warm zones 9 and up, no care is needed for water lily, tropical or hardy.  The plants that need specific attention for most regions are floating pond plants: such as water hyacinth, water lettuce, and frogbit.  Many water gardeners choose to buy these plant each year instead of wintering over, but wintering over can be easily accomplished with little effort by protecting plants from mildew, frost, and freeze, or by using a Garden Light Box. 

In zones 8 and below, a Garden Light Box is an easy way to winter over floating plants indoors.  All you need is a fish aquarium, mirrors (that can be purchase cheap at a glass store) and a sunny window.  The key to the plants surviving is keeping in the humidity up with a plastic wrap cover, warmth, and sunshine reflected with the mirrors.  It also looks really cool! 

A Greenhouse is a Wise Investment
This year, I invested in a small walk-in green house with plastic windows for my tropical and floating plants.  It will be heated with simple seedling heat mats.  However, greenhouses in sub-zero climates need thick double pane glass and possibly a kerosene heater.  I recommend purchasing your greenhouse from either Harbor Freight Tools (where I purchased mine) or Menard's Hardware.  Also check out greenhouse designs on Instructables.com. 

Wintry Conditions: Frost is a killer. When water lettuce freezes it does not recover. When there is a threat of frost or freeze, water lettuce should be covered or brought indoors. However, remember that they do need sunshine during the day.

Mildew: It is wise to cover your pond to protect the water and plants from freezing. However, covering your pond will also promote mold and mildew growth on your water lettuce plants. This is also a killer and your water lettuce will simply rot away. If you plan to winter over your water lettuce. An indoor water garden lightbox is a good choice, or install a fan to circulate the air and keep the mildew away.   Read Article: How to Grow Water Lettuce

Build a Garden Lightbox

This really works! 
In the cold seasons tropical plants need extra care.  Some pond plants (such as frogbit) will grow just fine with a regular florescent light and others will survive outdoors even in freezing temperatures.  Other more tropical plants need a little extra help with lighting, heat, and humidity.  Water hyacinth is one such plant, and a garden light-box can help!  Read Article: How to Make a Lightbox

How to Winter Over Tropical Water Lily

It is good for the lilies to go through 1 or 2 frosts. The cool weather helps force dormancy. After the first or second frost remove the plants from the pond. Put the pots in a cool but protected place such as the garage and let the pots dry out somewhat until the soil is barely moist. Wrap each pot in a garbage bag and close the top of the bag so that they will not dry out much more.  Do not close the top tightly; this way the plant will be able to breathe just a little. Sealing up the bag tight will also promote mold and mildew which will...   Read Article: How to Winter Over Tropical Water Lily