The definition of the SSO should be viewed in conjunction with SOLAS regulation XI-2/8 on “Master’s discretion for ship safety and security”, which makes it clear that the master has ultimate responsibility for safety and security.
The phrase “accountable to the master” in the definition of SSO is intended to cover those situations, for example on large passenger ships, where the SSO is not the master, by reaffirming that the master has overall responsibility for security. There is implicitly no intention of preventing the master from assuming the duties of SSO, as this would be inconsistent with SOLAS regulation XI-2/8.
It is, of course, for the national Administrations to decide if they wish to impose particular restrictions on who may serve as SSOs on ships flying their flag. This should, however, not be imposed by national Administrations on ships not flying their flag through port State control measures, since this is clearly the prerogative of the Contracting Government of the flag State concerned.