They look as though they are related to Eichornia crassipes
, the water hyacinth, but they turn out to be Heteranthera
species, and there are two of them. The first is, I think, H. reniformis
This species has multiple flowers on a stalk. The other plant in the picture is Ludwigia peploides:
The other species has single white flowers. It could be H. limosa
, which usually has blue flowers, but is also shown in a number of pictures with white flowers. Or, it could be H. rotundifolia
, which, on the USDA plant database, is west of Mississippi but not in Mississippi.