Japanese Iris (Iris ensata), Lobelia siphilitica, Cyperus papyrus, Star grass, Epipactum gigantea (Stream orchid), Mimulus cardinalis, Hibiscus moeschuetos to name a few. Take a look at pond plants. There are many marginals which would work just fine.

wanda

Joe Pye weed (Eupatorium) is another good one if you need something tall.It blooms all summer long.

Find a book on pond plants and try anything recommended for bog gardens. I use astilbes, daylilies, creeping jenny, chameleon plant, and siberian iris. I have some hardy hibiscus back a bit where it's not quite as soggy.