Hi Garry. I used to keep my waterlilies in cold room in my basement before renovations when cold room was removed as gave me a 8 x 12 foot area in the basement. Anyways all I did was remove any dead leaves and stick the pot in a laundry tub and check regularly to make sure the pots didn't dry out. My cold room was always dark as rarely went in there. I also overwintered some in my mother's cold room. I pulled the plants from the pond and put them in big plastic garbage bags, tied the bag shut and put in her cold room. Never checked them all winter. Both methods worked fine. In the spring (April here) I removed them and put the laundry tub on the patio outside filled the tub with water treated with Prime and let them grow. By the time they were moved into the pond which here is not till late May they had good growth and could be lowered to the bottom of the pond. Now that I don't have a cold room I leave them in the pond and they overwinter fine but growth is very slow especially the last few years as our springs have been cold. My son bought a house with a cold room and I think I'll overwinter some of my lilies there this year as it's nice to have that plant coverage early in the spring plus it's a very cold job to get those lilies moved up to warmer water when the pond is only 40F or less.

I live North, pa I leave mine in pond year round , theyre fine every year.

Ponds are slow to warm up in Spring, if kept indoors with a bit of light to encourage foliage you would get a head start if the light is good and temps are in the fifties compared to the chilly 40's at the bottom of a pond