Nothing in my books says anything about aquatic cannas, Emma but, given that hostas are amazing at surviving both in very dry and very wet conditions, assume what you saw in the garden centre have been cultivated to deal with it. Also, there was a recent question about lobelia cardinalis, the red one with purple leaves, being suitable for a a marginal. Was interested to see Marg's reply, advising against it as a marginal as I had used it as such in my pond with no probs. However, I can only assume it is the variation in temperature in different parts of the country. Being in the South East, a little more south than you are, can only assume better shelter than where Marg is on Lincs/Cambs border.
If it were me, I think, having raised a few cannas, I would gently try to aclimatise one or two to experiment. After all, nothing ventured, nothing gained, and you are nothing if not intrepid! Go for it and get your own answer then, next time question comes up, you will have an answer! :D
