The shrubs discussed on Page 2 can be attractive, but I've saved the best bloomers for last -- specimens desirable as water garden plants.
Wild bergamot (Monarda fistulosa) is a close relative of a perennial sold at nurseries: bee balm (Monarda didyma). It is a fine choice for water garden plants in hardiness zones 4-9. This member of the Mint family bears lavender (most commonly) blooms in July and August. The flowers are tubular and grow in rounded clusters. Wild bergamot likes soil that is slightly acidic. Height up to 4'. Grow in full sun to partial shade. Wild bergamot is widespread in the U.S., being indigenous to every state except Alaska, Hawaii, Florida, Washington, California and Nevada.
Marsh marigolds (Caltha palustris) are an early-spring bloomer. If you've ever been hiking through the woods in spring and encountered their cheerful yellow blossoms while traversing swampy ground, it won't surprise you to hear me proposing marsh marigolds as potential water garden plants.
If you need water garden plants of a taller stature, Joe-Pye weed may be a good choice. It can attain a height of 6 feet and bears mauve-colored flowers. If you have a large area to fill in, you may wish to grow Joe-Pye weed the way it grows in nature, namely, in masses.
Finally, use cardinal flowers (Lobelia cardinalis) as water garden plants if you crave a showy scarlet-red bloom that will turn heads. Its tubular flowers grow on spikes. Bloom time ranges from July to September. Cardinal flowers have been known to reach 4' if grown in the sun; in partial shade they stay shorter but are still attractive specimens as water garden plants. Cardinal flowers are native to all of the lower 48 states in the U.S., except for Washington, Oregon, Idaho, Montana, Wyoming and the Dakotas.
The USDA Plants Database has been helpful in compiling this list of native specimens suitable for wet areas and for use as water garden plants, as has reader, Al Dubovick.