Typical Spoonbill behavior of stirring and sifting the water or bottom.
Like so many of these larger wading birds the prey they actually live on is tiny. The diet is small minnows, shrimp, and insects.
Of note here is the depth of the marsh compared to the size of the bird. This is as deep as a Spoonbill will hunt. Ant higher and they will move on since they can’t fish.
This is the exact reason Roseate Spoonbill disappeared from their breeding grounds in the Everglades. Man made changes to the water levels almost wiped out all the Florida Spoonbill.