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Keyed in multiple places:

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Key to Hydrocharis

Hydrocharitaceae

Hydrocharis

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1 Petals broadly ovate, > 1.5× as long as the sepals; filaments fused for < 1/2 their length; larger aerenchyma (as measured at 1 mm from the midvein on the underside of the leaf) 0.1-0.5 mm across; stranded plants with deeply cordate leaves similar in shape to the floating leaves, with strongly arcing primary veins
1 Petals linear, < 1.5× as long as the sepals; filaments fused for > 1/2 their length; larger aerenchyma (as measured at 1 mm from the midvein on the underside of the leaf) 0.4-1.6 mm across; stranded plants with leaves with obtuse, truncate, or subcordate bases and all major veins gently curving and parallel
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Key C1: floating aquatics

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1 Individual leaves > 2 cm wide.
  2 Leaves obovate, cuneate at the base, sessile, pale green; plants floating because of “unwettable” leaf surfaces
  2 Leaves orbicular, cordate or truncate at the base, petiolate, dark green; plants floating because of petioles inflated as bladders, or inflated cells centrally located on each leaf.
    3 Petiole terete, not air-filled; plants floating because of inflated cells centrally located on each leaf (most easily seen on the lower surface)
    3 Petiole conspicuously expanded into an air-filled bladder; plants floating because of petioles inflated as bladders
1 Individual leaves or “fronds” < 2 cm wide, or leaves absent.
      4 Submersed portions of the plant with small (< 4 mm in diameter) bladder-traps (Utricularia) or snap-traps (Aldrovanda).
        5 Plants with snap-traps present on leaf tips; petioles with 4-8 laciniate bristles; leaves whorled, articulated basally into snap-traps; [uncommon non-native, VA and NJ in our flora area]
        5 Plants with bladder-traps present throughout the plant (rhizoids, stolons, and/or leaves); petioles lacking laciniate bristles; leaves alternate or basal (look at entire floating stem); [widespread natives]
      4 Submersed portions of plant lacking small bladder traps.
          6 Plants dichotomously forked, upper surface of leaves velvety or variously hairy
          6 Plants unbranched, or if branched, irregularly so; upper surface of leaves glabrous, waxy.
             7 Plant thalloid; flowers embedded in the thallus in reproductive pouches
             7 Plant with well-differentiated stems and leaves, the leaves obviously and alternately spaced along a well-developed stem; flowers axillary