There are some moisture loving pink coreopsis, this *could* be something in that clan.
I'm anxious to see what others say.

The only pink coreopsis is Coreopsis rosea. It is widely cultivated and naturalized outside its native range.

Cool. Thanks for the steer dbarron. I worked through Weakley's Coreopsis key and found two pink-flowered candidates - C. rosea and C. nudata. Per the FNA descriptions, the later is a relatively tall, water loving plant that blooms in the spring. Coreopsis rosea is much shorter, and flowers Aug-Sep on marsh edges. If the genus is Coreopsis, then I'm fairly confident that the plant is C. rosea. However, the BONAP map shows that C. rosea is a native species that is rare, and only known in a few eastern states. So I am also curious to see what others say.

Hmm, as far as the nativness of it...I wouldn't worry much about that. There's always been a fair amount of illicit transportation of water organisms in boat bilges and such. If fish get transplanted, so can water plants :0
Yeah, it looks more like rosea of the two to me...but again, I don't know.