Comment:  I think this is weird and kinda useless...haha.  But it does seem really fun.  The pictures that I'm posting are just from the Vanuatu Post... They make me wanna go send some waterproof postcards, haha.

Article:
Though not common, there are five underwater mailboxes for you to visit:
1. Vanuatu Post created the world's first underwater post office. Situated within a marine sanctuary off Hideaway Island, this post office, staffed by a postal employee, can receive mail if you are a visiting diver or snorkeler. The best part? The lines are always short.

2. On Japan's southeastern coast, there's a mailbox 33 feet beneath the surface of the water -- the deepest mailbox on record. Each day, the contents are collected from the box, which reportedly contains as many as 200 pieces of mail. Conveniently, the group who developed this post office also invented Surumail: edible, squid-flavored postcards.

3. Malaysia's Reef Dive Resort has set up what it believes to be that country's first underwater mailbox. Any mail sent through this mailbox is sealed inside a plastic bag and postmarked with a special stamp.

4. St. Thomas, USVI, has an underwater post office inside its Underwater Observatory, which means you can get the "underwater stamp" without getting "underwater wet."

5. Paradise Island, in the Bahamas, boasts an underwater mailbox adjacent to a spiraling 100-foot tower.




   




Source: Gadling, Vanuatu Post

Is that mean they ship to the underwater houses?

Not too sure about that.
To send the mail, you can drop them off at the underwater mailbox though.