Can something be air tight but not water tight?

No. Air tight means that no air can pass through and water may be able to, but water tight means that no water can pass through and air may be able to.

The only way maggots could get into a truly air-tight refrigerator would be if the flies were already there in some form - eggs, larvae, pupae, adult flies.

No passenger vehicle (that is not for flight or underwater use) is completely airtight. If they were, you would asphyxiate in your own carbon dioxide.

I don't know, have been looking for that for ever.

Not too different from bulkheads which can be sealed off ( to prevent flooding) in a modern submarine. Below decks, some were closed all the time.