What a great thread idea!
My favorite resources for identifying the ranges of native species for both land- and water- plants are these:
The Biota of North America: North American Plant Atlas
2014 BONAP North American Plant Atlas
And if you live in the NYC Area:
The New York Metropolitan Plant Atlas
I use them by looking up the genus of a species I'm wondering about, and looking through the species list until I see the one I want. NAPA lists, county-by-county what species is native (light green), adventive/noxious (blue/purple), and rare or endangered (yellow/orange). If it's native to a state, the rest of the state will be dark green, if it's invasive, the state will be dark blue.
For the NYMPA, it's the same thing but even more specific than the county-level (ie, for Ludwigia palustris, it is native to Brooklyn is only located in Mill Basin and Prospect Park, and the map reflects this). Green squares are sightings from the Brooklyn Botanic Garden's 1999 survey, yellow are historical sightings. You have to read the label to learn if a plant is native, there's no color code for that. Please don't use this info to collect or release plants: in most waterways, it is illegal, and there are many species that are endangered here due to habitat loss as massive degradation and some areas have species that are endangered everywhere (not too many coastal plains still exist anymore).