> The coldest is a bit after dawn.

The building (and deck surface) will continue to radiate heat throughout the night on a clear night, especially with few obstructions like a waterfront building has. That's how crops can frost with airtemp > freezing, and why the guidance is to cover your plants. It's not to insulate them against convection.

Counterintuitively in this scenario, this effect is greatest when the wind is calm.

https://en.wikipedia.org/wiki/Radiative_cooling#Architecture