What ever he is ordered to do.
Lieutenants (along with Ensigns and Lieutenant Junior Grades) in the Navy are typically Division Officers, meaning they are responsible for a unit of men that have a particular function. They may be a Department head, responsible for several divisions.
They may even be in command of a very small ship or boat, or even a set of boats. The last Commanding Officer of the US Navy's last Asheville class Patrol Gunboat was a Lieutenant.
Aggie80