To Sail a Darkling Sea
Book 2 of the Dark Tide Rising series.

This is the continuing story of rebuilding the US military (mainly Navy/Marines/Coast Guard) after the release of a bio-weapon turns people into "zombies". They are not zombies, though. They are dangerous, infected people with damaged minds who have turned animal. The main characters are Faith "Shewolf" Smith, a 13 year old girl who is provisionally granted a commission as Marine 3rd lieutenant and her 15 year old sister, Sophia "Seawolf" Smith, a provisionally commissioned ensign who captains a small navy vessel.
US Navy, cruise ship, US Marines, salvage, zombie apocalypse