One thing that would survive almost anywhere in water is the American Lotus (Nelumbo lutea). They would need to be in the bottom end that never dries out, but they would bloom all summer, they are beautiful, large and the big yellow flowers smell lovely.
I see you already have some iris, and there are many other kinds of those that love living in water, and are great for filtering the water, too. But the more tender kinds like Louisiana Iris are not hardy.
Cannas are another plant that does great in water, and blooms like crazy all summer. Again, not winter hardy though, unless you are in the South.
You still haven't told us if you live in Alaska or Georgia? Montana or Texas? Maine or California?