Galveston Bay is a bay in the western Gulf of Mexico along the upper coast of Texas. It is the seventh-largest estuary in the United States, and the largest of seven major estuaries along the Texas Gulf Coast. It is connected to the Gulf of Mexico and is surrounded by sub-tropical marshes and prairies on the mainland. The water in the bay is a complex mixture of sea water and fresh water, which supports a wide variety of marine life. With a maximum depth of about 10 feet and an average depth of only 6 feet , it is unusually shallow for its size.