I've never see it before - I can't wait to hear if you identify it!

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Flower and foliage appear to be Bulltongue arrowhead, Sagittaria lancifolia.
A native aquatic plant species that can be found along the East coast from DE & MD - southward and along the Gulf coast to TX & OK.
Not as common as the Broadleaf arrowhead, Sagittaria latifolia, which can be found in 48 states and most of Canada.
Rb

Here is a link that might be useful: Sagittaria lancifolia