This is pretty cool, and has inspired me to make a trip (does anyone know if you need training to snorkel in Japan?). One can send mail underwater off the coast of Wakayama.
on Japan's southeast coast, there exists an underwater mailbox. It's not a sunken relic that now serves as an artificial reef. Rather, it's an actual, working mail collection point officially recognized as part of Susami's postal system. Evidently, the mailbox is 33 feet beneath the water, and divers are required to use special waterproof plastic postcards. Each day, the contents are collected from the box, which reportedly contains as many as 200 pieces of mail on the busy days.
Not that far away, 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 plastic bags and is postmarked with a special stamp.
Read the whole article.
And while you are there, why not use an edible, squid-flavored post stamp?