I grew up swimming and sailing on Buzzards Bay. By the end of August it can be like bathwater. The bay heats up more efficiently and holds the heat longer.
As a kid I remember people saying there’s a side branch of gulf stream water that flows into the bay and helps it heat up but I don’t know if that is true or not.
I will say this, the beaches in North Falmouth and Cataumet are amazing in September. There is nobody there and the water temperature is still in the 70s.