Water poppy (Hydrocleys nymphoids) has yellow flowers and rounded leaves. The flowers are about 2" in diameter and spreads over 1-6 feet in your pond. It can be restrained by clipping off some of its runners. It will grow well in a lot of shade, but prefers the sun. It should be planted 4-12 inches deep. Once your water temperatures reach 70 degrees it will continue to grow and bloom until killed by the frost. An excellent small flower for bogs and shallow pond areas or containers.
This picture was taken in my sister's pond.
Buy water poppy, tropical bog plant here.