1st Lt.

[Cecil DP]

Also remember that a Marine Captain (Or any other Captain, O3) while serving on a Navy ship are given a "courtesy promotion" to Major, as a Ship has only ONE Captain...

I have frequently heard the Skipper of the Ship's Marines called or referred to as Captain of Marines, but I have never heard one called Major.

No never.  Frocking went out a long time ago.  In fact, frocking officers were restricted to Naval and Army Officers.  Marines were not considered "important enough", nor enough of them around to "promote then demote".

Frocking and the courtesy promotions are two different things. The "courtesy title", not a promotion was used in a time in which there was 1 Captain on a ship, and to avoid the problem of which Captain gave an order , The Captain was the one and only. If there was another Captain on board IE Flag staff, that officer would be refereed to a Commodore. In today's Navy they still have only one Captain, if you were aboard the Enterprise or any carrier the O-6 commanding the air operations will be refereed to as the "CAG" or Carrier Air Group Commander, never Captain.

Frocking is investing an officer who has been selected for advancement, and is serving in a position which calls for that grade, with the title and insignia, ahead of his actual appointment date. He's still a Major for pay, but allowed the uniform and title until it's official.

The reference's to Heinlein's use of the titles were probably correct prior to the end of WWII. Heinlein was an Annapolis grad and I'll take his word based on that. (About the only word I'll take from an Annapolis, West Point, or USAFA grad).

[JCJ]

Much off topic, but why doesn't the Marines follow the same rank titles as the Navy since they are under the department of the Navy? I mean this from an officer standpoint.

Because the Marine Corps is a military service :-)

Seriously, in general the normal daily activities in USMC units are often carried out by Marines who are a rank lower than would be the case in a sister service (i.e. sometimes a responsibility that would ordinarily be assigned to a USAF Major would be assigned to a USMC Captain).  The Marines take some pride in this.

[SarDragon]

BTW, the Navy also frocks enlisted folks, particularly E-7 through E-9 selectees.

[Grumpy]

Well, that's the frocking Navy for ya.