Until the 18th century it was called “the Washes” from the various rivers and streams that “wash” slowly into it from the extremely flat and featureless surrounding land. These include the Great Ouse, (which is a lesser river than the Ouse in Yorkshire!) the Welland the Witham, and the Nene. These have deposited so much silt over the centuries that medieval fishing villages and ports are now miles inland. One of these is Boston on the Witham 5 miles from the coast now, but in the early 1600s a thriving port, a centre of Puritanism and the setting off point for many of the early Pilgrim Fathers including the founders of Boston, Mass.
In 1205 King John was caught by the tide whilst crossing the Nene and lost his baggage train, including the crown jewels. They are still under there somewhere. As Sellers and Yeatman put it in "1066 and All That": “John finally demonstrated his utter incompetence by losing the crown and all his clothes in the Wash”.
Reference: “Brewer’s Britain and Ireland”