What were they eating

Thursday afternoon I was driving along the shoreline in Poquat( N.setauket). The tide was very low and a 1/2 doz. seagulls where swimming in about one or two Ft. of water. Then they would fly-upward about 2 feet and dive under the water and pull up a tasty treat.
They were not clams, they were very dark and soft bodied looked about 3 inches.
I was thinking maybe sandworms, anyone have any idea?