I'd 2nd the marsh marigold and the elodea densa.
Other nice plants are the native loosestrife (Lythrum salicaria) but it does grow really tall (5ft easy), but bees will swamp it. however grown in a pot it is prone to blowing over in the wind, too big I'd say for a bath, but planted outside behind in a damp spot may work.
Their is a native bog arum, Calla palustris, but while it has typical white flowers, the berries are poisonous, do not eat them.
The native flag iris is nice, but grows tall again & spreads fast, so will have plenty of baby plants to give away, the varigated form may be less vigorous.
Iris ensata are nice, less vigorous, but not native, great flowers with some of the varieties like laughing lion.
I would avoid any of the native bulrush, they grow big, spread fast & have very strong roots which I have read can puncture liners in ponds.
Carex pendula looks nice near water, pendulous sedge, grows big and likes to spread via rhizome type roots and seeds well too.
Diarama look nice with flowers drooping over water, but not in the water itself.