I'm giving "fanal astilbe" a chance in my waterfall. I only added it a few days ago after discovering it could tolerate wet soil. So far it seems to be adapting really well, and it's a very beautiful plant.
I feel like the pictures of these plants I've found don't quite do the colors justice--but maybe I've just got a bad computer monitor. The leaves are a deep green, and the flower is a rich magenta/fuscia. They say that if you leave the flower stocks rather than cutting them down when the flowers fall off, the stocks can add visual interest in the fall.
As I said, I only added it a few days ago. It's not in an ideal location either, as up on the waterfall its constantly getting splashed. However, it does seem to be doing well.
I took the advice I found on here and added a Stella De Oro Daylily, although it's a daylily hybrid and I wasn't able to find anything about it tolerating bog conditions like other daylilies. However, it also seems to be doing well. It's flowering a the moment, and all of the foliage appears healthy.
In one of my ponds, I have Virginia creeper that is creeping its way into the pond from the land. I am surprised how successful the underwater runners are (though I wouldn't say the plant is thriving underwater by any means). In previous years the creeper that went into the water just died and rotted very quickly. I wonder if it's possible that it is slowly adapting to the conditions a little more each year? There are some plants that can be "trained" to be emergent after all.