I would think the temperature over the water would stick very close to that of the water.
I remember driving from upstate Rhode Island back to Newport on an 80-degree day one early May. As we came down the hill toward the Jamestown Bridge, it was like driving into an air conditioner! Sea temp. was still below 50 degrees.
Same experience sailing on a warm sunny May day -- until the breeze kicks in and the air temp drops to that of the water.