I perhaps didn't make it clear that here in the UK it is rare to find lilies that will grow in more than 3-4 ft of water apart from the common white form of Nymphaea Alba and our native yellow Nuphar lily. As Meyer has also noted, these are species lilies and not hybrids.
I'm not aware of the types available to you in the US, so you may well have many more deep water ones. However, as WaterLilly is from Azerbaijan, I've no idea what may be available there.
In answer to your question WaterLilly, in my opinion, elodea IS one of the best, but remember my pond is 2.5 ft max in depth, I don't know what depth it is able to tolerate. However, when I pull it out, some lengths are easily 5ft long so who knows what length it can grow to given the right water conditions. I've not grown hornwort for along time as I've only seen it for sale the once and it didn't last very long for me, I don't know why though!