The rank insignia on Ryan's shoulder indicates that he is a Navy Lieutenant Commander.
The only reason Charlie Sheen agreed to do this episode was because he lost a bet to Matt Leblanc. Charlie and Matt were up for the same role in the baseball movie "Ed". Charlie bet Matt he would get the role. The role had already been offered to Matt, unbeknownst to Charlie, so he told Charlie that he would take the bet only if Charlie agreed to do an episode of Friends if he lost. Matt got the role and Charlie did this episode.
The Chicken Pox vaccine became available in the US in 1995 and became mandatory for all military personal who did not have a medical history of prior infection. This episode aired in 1996, meaning Ryan would've had to have receive the vaccine if he wanted to A) stay enlisted and B) still be allowed to work in a submarine (confined area with high rate of person-person contact).
Ryan visits Phoebe in Monica and Rachel's apartment instead of in her own apartment, which she had moved into by the show's pilot, The One Where Monica Gets a Roommate (1994). Phoebe lives with her grandmother and states she doesn't want to pass her chickenpox onto her.