AFAIK the only country that BA operate into that demands crew have a formal printed visa stuck in their passport is the good old US of A, so if you end up going onto a fleet that visits the States you'll have some admin to do during the conversion course and a visit to a nice embassy near you. Every ten years you have to jump through the hoops again.
Most everywhere else it's done a clearofconflict describes, you gain entry with ID and/or Passport by the simple fact that you are on the General Declaration and in the case of the PRC, the company submits your name on the quaintly titled waterguard list....