Most potted water garden plants (hardy water lily, lotus, sweet flag, water plantain, cyperus) will overwinter well in a dark, cool spot like an unheated basement. Set each potted plant into a bucket with a few inches of water in the bottom. The plants will go dormant, so you'll have to clean up the leaves as they collapse. In the spring they'll send up new growth as soon as you move them into a brighter location. Floating plants in your water garden (including water hyacinth and water lettuce) won't come through the winter; they'll rot. You've got to treat them like annuals--toss them out in the fall and buy new ones in spring.