A few ideas for your floating island...
Canada Anemone, Anemone canadensis
Swamp Milkweed, Asclepias incarnata
New England Aster, Aster novae-angliae
Bottle Gentian, Gentiana andrewsii
Wild Iris, Iris versicolor
Prairie Blazing Star, Liatris pycnostachya
Cardinal Flower, Lobelia cardinalis
Great Blue Lobelia, Lobelia siphilitica
Monkey Flower, Mimulus ringens
Wild Bergamot, Monarda fistulosa
Crimson Beebalm, Monarda didyma
Obedient Plant, Physostegia virginiana
Blue Vervain, Verbena hastata
Ironweed, Vernonia fasciculata
Culver's Root, Vernonicastrun virginicum
And, they're all native like full sun, wet soils and are adapted to your northern climate.
