Did you have an occupational health check by your employers before you returned to sea duties? That would have defined whether you are fully fit to return to normal duties, or whether lighter/different work was recommended.
Re the vacancy on ship 2, is that being advertised as a vacancy so you would have to apply against other people? Your company can't stop you doing that but I believe that can decline a transfer, particularly if it might be more difficult to fill your job on ship 1.
Don't resign, that gives you no rights at all, unless you can prove constructive dismissal. Are you in a union or do you have a staff representative? That to me would be your best route, seek their advice first - they know the score and will either tell you that the company is right, or that they will help you to resist the change.