I think that you will find that most lenses are definitely not air tight otherwise you would run into problems with zoom and focusing actions that extend or retract parts of the lens due to the changes in pressure that would be induced inside the lens by the change in its volume. The better quality lenses may be dust proof but the only way to keep out water vapour would probably be to go for an underwater design, such as one of the Nikonos cameras, where the camera back, controls and lens are sealed with ‘O’ rings and all the moving parts of the lens are within the lens body, ie the front of the lens does not move in and out so there is no change in volume of the lens.
I've never had problems with moisture inside a lens but then I’ve never worked much in extremely high humidity. Keeping the lens and camera in a bag with silica gel should dry them out, and then avoiding opening the camera back, zooming the lens etc as much as possible should cut down the amount of water laden air drawn into the lens and camera. Try to avoid condensation, usually on the outside of the equipment, by not taking the camera and lens directly from a cold environment, such as an air conditioned room, into a warm humid atmosphere. When going from a warm humid environment into a cooler one the moist air in the equipment will chill and cause condensation on the inside of the gear; take the lens off the camera body to minimise this condensation.
If mould is becoming a problem then keeping the lens in a bag with a few drops of a volatile biocide may halt or inhibit mould formation, otherwise it’s off to a repair shop to have the lens stripped down, cleaned and rebuilt.