plants with vaguely oval leaves in ponds

 Arrow arum has triangular leaves with main veins along the middle to each point.

Broad leaved arrowhead has curving parallel veins in three bunches, aiming toward each point but without a much larger central vein (as in arrow arum)

Pickerelweed has veins almost not visible and lower lobes much smaller than in either of the above. It also nearly always has flowers all summer.

Spatterdock also holds its large leaves above the water as in the other three, they are larger in spatterdock and somewhat rounder, with the veins at nearly a 90-degree angle to the midvein of the leaf, over nearly the whole surface.

A "spatterdock" with leaves all floating and the bottom lobes (at the notch) overlapping one another is variegated pond-lily.

American lotus is rare but present in the area, has the largest leaves of all, holds them out of the water. they are entirely round with the stem right in the center as an umbrella. pale yellow flowers. Same thing with pink flowers is the planted sacred lotus.

All of which have larger leaves than the following:

Fragrant whtie waterlily has floating basicaly round leaves with a notch, much like "Pac-man".

Wild Calla has round leaves with a notch and a definite point, about 4 inches across, with not very obvious veins that diverge from the midvein and curve to follow the edge of the leaf, a lot like a small and rounder pickerelweed. 

Common Water-hyacinth has kidney shaped leaves with veins very curved and following the edges of the leaves, held out of the water, with a huge, swollen node in the leaf stem right at water level. This is not that.

Kidney leaved water-plantain has kidney shaped leaves somewhat above water level, only up to 3 inches across (if that) with wide stems but no swollen nodes.

Frogbit (there are american and european) has very round floating leaves about 2 inches wide with sometimes overlapping lobes at the notch with a central vein and generally two side veins that curve to meet at the tip and side veins barely visible off all three at 90 degree angles. Weird. leathery. 

water shield has floating oval leaves about 2-3 inches long.

water fringe has floating round leaves about 2-3 inches with a closed notch and small yellow flowers.

floating hearts have 2-inch floating heart shaped leaves. The "little" one is southern and has mottled leaves.

Pondweeds can also have floating oval leaves about 5 inches long by 1 inch wide.

water chestnut has diamond shaped leaves with ruffly teeth on two sides and they grow in very round clusters floating in the water. roughly 2 inches across. 

water pennywort has 1 to 2 inch round leaves with a notch and very definitely crenate lobes all around it (maybe 8 lobes per leaf). 

water lettuce has wedge shaped leaves in clusters like a green flower, each leaf with veins parallel and straight and all from bottom to tip of leaf.