Thanks again for the info.
I had not come across Alisma subcordatum Raf. or Alisma triviale Pursh since they are not well distributed in upstate NY (less than 15% of the counties) and not listed for my county by the New York Floral Atlas, nor for the counties with lakes we have visited most. These are not growing in their normal habitat on the perimeter but are on an island almost 2' deep and appeared last year as small floating leaves, much like American pondweed, but they were basal not having branched stems.
Thanks again.