What does SS on a ship stand for?

SS is short for Steam Ship and used on civilian ships. NS would be Nuclear Ship and there was at least one civilian nuclear powered vessel.

USS is a prefix designation for a military ships that stand for "United States Ship".

HMS is a prefix designation for a British ships that stand for "His(Her) Majesty's Ship".

Other ship designations you may see are 'MS' which stands for 'Motor Ship' and 'M/V' which stands for 'Motor Vessel'.