Can any m.d./d.o. become a cruise ship doctor regardless of their specialty? for example, could a general surgeon practice cruise ship medicine?

I would guess that most cruise companies wouldn't have many specific requirements other than MD or DO degree, appropriate licensure, and so on. But that's only a guess. Ask the cruise company!

During a cruise most problems which arise are non-surgical in nature so any doctor who thinks he is proficient in general medicine would be appropriate

Certainly it's an internal medicine job, but remember also that you are isolated with and limited to, all the tools that your infirmary can provide. Your access to supportive services is limited by location. There is a significant public health portion of this job which may involve quarantining. Apart from moments of horror, it should be a very nice job otherwise.