nobody knows?

I don't think you can overwater *any* canna.

In a bog, you might have to lift some varieties in the autumn to keep them from rotting, but they should bloom beautifully in season.

That's what I'm thinking. I've planted the Stuttgart and it seems happy - at least, it's finally putting out a new leaf. I also put in a couple of Hedychium forrestii that didn't have a home (apparently they like moist soil too). And yes, I'll definitely be lifting them in autumn!
Thanks for your reply.

You may have been thinking of the Longwood Hybrids. They are truly aquatic cannas.

Here is a link that might be useful: Longwood Cannas

Thanks for the link - that's interesting. I wasn't actually aware of the Longwood Hybrids, but was thinking of other Cannas such as 'Bengal Tiger' which can be used as pond plants...
http://www.plantdelights.com/Catalog/Current/Detail/00819.html

I think the consensus is that Canna glauca and its hybrids (e.g. the Longwood Hybrids) can be considered truly aquatic plants, but plenty of other Cannas can grow well in shallow water or boggy soil, as long as the temperatures are warm enough.

It is my understanding that one should wait until cannas have leafed out before putting them in the pond. That the rhizomes will rot if they aren't leafed out. That any canna is fine with this.

Good point.