While I was in Iraq, I used foot powder daily. Never had a problem with the boots not breathing or with them getting too soaked on the inside.

Not only does it help keep the boots fresh, but it also helps ward off fungal infection, even if it doesn't necessarily contain any antifungal agents. I used Gold Bond most of the time.

If you did get athlete's foot, then you'd need something that cures it cuz foot powder alone won't do it.

BTW, boot insulation works pretty good at helping to keep your feet from baking too.