Rob
Ansys Employee

For the solid, you can either ignore it and set a heat flux or temperature on the surface to the water. Or, include the solid and set a heat source to that zone. Both are valid, but you need to understand what each does. 

At the temperature you're saying, you can neglect boiling. Just keep an eye on the temperatures. 

Echoing Nick's comment. Whilst you want to see the air flow, run the first case full of water to see what's going on. It'll be steady state and run in an few hours (mesh & cpu dependent).  VOF will be transient, and could take several days to give a stable-ish result. 

You noted a lack of buoyancy. Did you set the density to be temperature dependent? Is gravity on? The steady model is a really good way to find all your problems, omissions and any mistakes as it'll go wrong/show silly values very quickly. You don't want to be waiting days to find you missed, or didn't think of, something.