There is a fully functional and submerged post office in Vanuatu that is one of a kind.
One of the world's only underwater post offices can be found off of Mele, Vanuatu, which hosts the Hideaway Island resort, a vacation spot specializing in scuba and snorkeling.
It's a fully functional post office, which means it also comes with the irritations of a regular post office. It's only open during certain hours, which are signaled by a flag raised above a float over the post office. You may arrive to find it closed, in which case you must take your mail back with you no dropping it off. Fortunately, though, you most likely won't have to wait in line.
And of course, because water would ruin any traditional postcards, scuba divers must purchase waterproof postcards and stamps and have them embossed on land rather than writing in ink. It all begs the question, why not just mail your letters from a post office on land? But that's not in the spirit of adventure.