You can put it in a bucket of water, but it will still need some light, otherwise it will just rot. A sunny window or a grow light might be beneficial. Overwintering a tropical water lily is not that easy, and a lot of people treat them as annuals. Others place theirs in a tub in a greenhouse. I've heard that some people put the tubers in the refrigerator, but I have never done that and have no idea how well it works.