This insures that the rootstocks will not freeze, killing the hardiest of aquatic plants. Waterlillies are safest since they are at maximum depth. Removal of dying dormant foliage is only maintenance required. Tropical lilies have their best chance of surviving the winter when placed in the warmest spot in the pond. Typically the warmest spot is the deepest spot, assuming no circulation is present.
Circulation can bring the cold water to the top of the pond and push the warmer, deep water away.
Plants that emerge out of the water like cattails, grasses and broadleaf bog plants should be trimmed back so their stems extend out of the water 1-2 inches. Trimming them below the water level tends to drown them. Bog plants like Iris that do not go very dormant in the Atlanta area should not be trimmed but allowed to grow as they please. Floating plants, like hyacinths and water lettuce, are target to freeze first. In some cases the hyacinth will overwinter but rebound very slowly the next year.