The Navy Dads Admin is correct. You will find the address at this site along with a lot of helpful information. Please note that if you send anything other than a letter you will have to fill out a short customs form, at the post office.
I my experience there is usally a link somewhere on the ship's website that gives a mail address. Usually addressed as
Rate, Last name, First name
Division
Ships Name
FPO AP with the ship's zip code
Replies
http://www.stethem.navy.mil/site%20documents/default.htm
I my experience there is usally a link somewhere on the ship's website that gives a mail address. Usually addressed as
Rate, Last name, First name
Division
Ships Name
FPO AP with the ship's zip code