Thank you Rob,
In the conceptual (tank-type) model I am using, the top wall represents the water surface level (please see snapshot below). So, I hooked the air temperature profile to this boundary. I did not create an external mesh representing the air zone. Do I need to include this zone in my mesh?

The lower part of the tank represents the sediment zone where there is no heat transfer with the outer part of the model (air).

The boundary between water and sediment has been configured as Interface. Is this the correct zones connection? If so, should I select "via System Coupling" as the thermal condition for this interface?

I would appreciate your guidance on this.

Figure 1. Model zones and water-sediment interface.

Figure 2. Water-sediment interface configuration.