No Communication With the Outer World
When you board a submarine, you should be prepared for the fact that communicating with your loved ones may not be as simple. Mail will not swim deep into the sea to retrieve yours to send to your family or vice versa (obviously). The only time you can deliver or receive mail is when the submarine surfaces.
Even so, the submarine emits a signal that reveals its position, which the crew does not want. So, keep in mind that communicating with the surface world while you’re on a submarine won’t be simple.