I toyed with the idea of overwintering some of my pond plants indoors, but when I discovered what it would mean in terms of work -- the water has to be changed every couple of days, so I've been told -- and the cost of the lighting equipment I would have needed, I found that simply buying new plants in the spring was much less expensive. Both water lettuce and water hyacinth are pest species in my area and very invasive in local ponds. If I wanted to look around a bit, I could probably get new plants for free.
If I buy the plants at an aquarium store, it costs about $10 to restock both of my ponds every spring. My ponds are just decorative, only about a foot deep, and freeze solid if we have a cold winter. This year wasn't that cold, though, and it looks like I won't have to bother restocking. In fact, it seems I have a bumper crop of both species, so I'll have to remove some of them with a rake. I figure they'll be a good addition on the compost pile.