An Undersea Mailbox

Today, I will talk about an undersea postbox in Wakayama Prefecture in Japan.

As the name implies, this postbox is located under the sea.

The distance from shore is 100 meters, and the depth is 10 meters, so you have to prepare diving to put a letter in the postbox.

An average of ten letters per day are dropped, and a diver collects them everyday, then they are delivered throughout Japan.

In 2002, this postbox was recognized in the Guinness Book of Records for the deepest underwater postbox.

By the way, since the undersea postbox is deteriorated by shellfish and seaweeds, it is replaced once a year.

海底ポスト

今日は、日本の和歌山県にある海底ポストを紹介します。

このポストはその名のとおり、海底に設置されています。

岸からの距離は100ｍ、水深は10ｍであるため、投函するにはダイビングの準備が必要です。

1日の投函数は平均10通で、ダイバーが毎日回収し、実際に日本全国に配達されます。

2002年には、「世界一深いところにあるポスト」としてギネスに認定されました。

また、海底ポストは貝や海藻によって劣化するため、毎年1回交換しているようです。

• The distance from shore is 100 meters, and the depth is 10 meters, so you have to prepare diving to put a letter in the postbox.
• The distance from shore is 100 meters, and the depth is 10 meters, so you have to prepare to dive to put a letter in the postbox.

Wow that's really cool!

• An Undersea Mailbox
• An Mailbox under the Sea

  This'd sound more natural for a title I think.

• Today, I will talk about an undersea postbox in Wakayama Prefecture in Japan.
• This sentence is perfect! No correction needed!

• As the name implies, this postbox is located under the sea.
• As the name implies, this postbox is located under the sea.

  If "undersea postbox" is the proper ("official") name of the postbox, then technically you should have made that more clear before referring to it as "the name".

• The distance from shore is 100 meters, and the depth is 10 meters, so you have to prepare diving to put a letter in the postbox.
• The distance from shore is 100 meters, and the depth is 10 meters, so you have to be prepared for a dive if you want to put a letter in the postbox.

  The difference between "prepare" and "be prepared" in this case is, I think, that in your sentence the meaning is literal (you have to prepare your equipment etc.), and in my correction it's more about being mentally "prepared". I'm not sure which meaning you had in mind.

• An average of ten letters per day are dropped, and a diver collects them everyday, then they are delivered throughout Japan.
• An average of ten letters per day are dropped, and a diver collects them everyday. After that, they get delivered throughout Japan.

  Fun fact: sometimes you can use "get" with passive to carry across a kind of feel that there is a process going on and it takes a bit of time.

• In 2002, this postbox was recognized in the Guinness Book of Records for the deepest underwater postbox.
• In 2002, this postbox was noted in the Guinness Book of Records for the deepest underwater postbox.

• By the way, since the undersea postbox is deteriorated by shellfish and seaweeds, it is replaced once a year.
• By the way, since the undersea postbox is deteriorated by shellfish and seaweeds, it is replaced once a year.

  "By the way" sounds weird here. Hard for me to tell you why, but in my opinion that phrase feels more at home when you're talking to someone, not so much for written stuff.
  
  I'd go for something a bit more neutral, like "Also," or something more narrative like "Another curious fact is that...".