Can a US Navy Ship Have Two People on Board Ranked as a Captain?

An aircraft carrier will likely have several O-6 assigned but only one will be "Captain". Carl Vinson had a Captain as CO, XO, Chaplain, Reactor Officer plus an air group commander or CAG and his deputy. There was also a Captain who was the Destroyer Squadron Commander called "Commodore" and I think two Captains assigned to the Admirals staff. So at the time there were probably seven or eight Captains on-board. Without looking in my cruise book I am not 100% but bet I am pretty close. I recall we had a Captain Operations officer for a while also.

Yes. Remember though that in the navy "Captain" is both a rank and an appointment. There can be two officers holding the rank of Captain aboard a ship but only one officer can hold the appointment of "Captain".

"Captain" is the person commanding the ship. On most submarines, esp SSN's, the "Captain" is normally a Commander. Of course, after a successful command at sea tour, promotion to the rank of Captain is sure to follow.

Sure! Why not? Simply having that rank doesn't mean that you are Captain of that particular ship.

When an admiral is at sea he is aboard a ship commanded by a Captain, who is Junior in rank.

15 to 20 years.

yes.

Captain is both a Ttile and a rank.

occassionally both the XO and CO of a carrier are O6s. while both are Captains, only the CO is THE Captain.

Yes. You will find several O-6s on an Aircraft Carrier in charge of different fucfunctions

Yes, or more than 2. But only one will be the CO of the ship

There can be MARINES afloat and they have a handfull of captains too. equal in rank to a NAVY LT.

0-3 NAVY LT

0-3 Marine CAPT

0-6 NAVY CAPT

0-6 MARINE COL