Which bay is about midway between the mouth of the Niagara River and the head of the St. Lawrence River?

Sodus Bay is located approximately midway between the mouth of the Niagara River and the head of the St. Lawrence River, making it the correct choice. Its strategic position along the southern shore of Lake Ontario not only provides it with access to the lake but also situates it within the region known for its settlements and boating activities. This bay is recognized for its natural harbor and recreational opportunities, contributing to its importance in local geography.

In contrast, other options either lie further away from this midpoint or are situated in different contexts that don’t align with the geographic descriptor provided in the question. For example, Port Bay and Irondequoit Bay, while part of the Lake Ontario shoreline, are located in different areas relative to the specified points, and Chaumont Bay is further to the north, closer to the St. Lawrence River, rather than midway between the two major river points identified in the question.