I am moving to Costa Rica and want to take both of my iMacs. Humidity and salt air are a problem. Will leaving the iMac running help to keep humidity out of the unit? I see only one vent; maybe a cover?
Humidity is not an issue.
Humid air on a warm/hot computer components won't be a problem. It's when the temperature of the components are lower than the dew point of the ambient air that it will become a concern; water will condense.
Running the computer won't keep humid air out as it's the ambient air that gets circulated. Unless your computer somehow gets colder than the room that it's located in, it will be fine.
(I live in the tropics with many, many 100% humidity days)