Technically, the Seaway is composed of two sections: 1. Welland Canal connecting Lakes Erie and Ontario, and; 2. The portion of the St. Lawrence River between Lake Ontario and Montreal. Downstream of Montreal, it's simply the St. Lawrence River although the navigation channel in this stretch is fairly narrow. Downstream of Quebec City, the St. Lawrence widens pretty quickly (as you noted) and once past Ile d'Orleans, it's much like a widening estuary to the Gulf of St. Lawrence.