When I first heard of this, I thought two things...
1. Wouldn't the letters get wet?
2. Who would actually use it?
Well, the answers to them questions are rather simple... the mail gets put into a waterproof bag/container, and deep sea divers would use them!
It is surprisingly more popular than I originally thought it would be, at the busy times it actually has as many as 200 pieces of mail! There's also a special stamp which is stamped on the mail - obviously - because a normally one wouldn't be waterproof!