In my zone (4-5), it is a perfect time to divide and plant my Water Lilies.
Find a pot with holes in it that is large enough for your rhizome to fit with a bit of room.
Put a Pond Tab or other Water Plant fertilizer at the bottom of the pot.
With your hand covering the newest growth, pour the gravel, clay, soil mix over your hand until it weighs down the rhizome. Pull up the head of the rhizome a slight bit and add a little more soil underneath it so that the head of the lily points up a bit.
The pot should look like this when done. Place the newly potted Lily in the water slowly. If you put it in too fast the soil might dislodge and the rhizome will float away and you'll need to do it again.
I recommend you put the pot at a depth which will be at least 6 inches below the bottom of the ice cap for your pond in the winter. Hardy Lilies are usually rated for USDA Zone 3, but if the rhizome freezes the plant will die. In my ponds the ice cap is about 8" deep and the pot is 6" tall so I put my pots at 20" deep.
Once a lily has become established I move it to a larger pot and into a deeper location. Due to our short growing season, I do seldom plant a lily more than 30". In warmer zones, larger liles can go up to 4 ft.
Remember to fertilize you lilies regularly throughout the growing season for the most blooms.