The temperature of water changes slower than the temperature of the air because water has a higher heat capacity. Heat capacity is a property of a material that describes how much heat energy is required to change the temperature of a substance by one degree Celsius. About one unit of heat energy is needed to warm the air one degree Celsius. Four times more heat energy is needed to warm the water one degree Celsius. (Look at the table below for the heat capacity of some common materials).
Heat capacity is one reason why the surface water temperature in Narragansett Bay reaches a maximum at the end of the summer. The air temperature may be 80°F or more in June but the water temperature in Narragansett Bay is usually 65°F. The air temperature can also change a lot during the day but the water temperature doesn't change as much.
What are the other reasons Narragansett Bay doesn't reach its maximum temperature until the end of summer?