You cannot do justice to The Hermitage in a week much less a few hours. Your guide will give you just a taste of the treasures within.
I had an extended tour (several days in fact) while there one March and it was crowded even then so prepare for crowds during a summer cruise/tourist season. The massive museum gets very hot in summer and don't be surprised to see open windows within feet of a Raphael.
On our stop there while cruising, and my second visit to the city, I arranged with our SPB guide to start our tour with the grand masters. Our Roll Call group was well versed in art and had prioritized our itinerary. The suggestion to go online and familiarize yourselves with the museum is a very good starting point. Pick the exhibits you wish to view and arrange this with your guide if you are using a private tour company - which is the very best way to see the most sites in St. Petersburg.
Have a wonderful cruise.