In the USN (Submarine) what do these rates or ranks mean?

XO Executive Officer (Commander pay grade of 0-5)

PCOB? but FTCM is a Master Chief Fire Control Technician (pay grade E-9)

COB Chief of the Boat is an enlisted sailor on board a U.S. Navy submarine who serves as the senior enlisted advisor to the commanding officer and executive officer, and assists with matters regarding the good order and discipline of the crew. The Chief of the Boat is the submariner equivalent of a Command Master Chief in shore and surface units. There is only one COB on a submarine and he is generally responsible for the day-to-day operations of the non-nuclear-trained portion of the boat, the morale and the training of the boat's enlisted personnel. The COB is typically the most senior enlisted man; however, the commanding officer is neither required to select the highest ranking sailor nor the most senior in grade or time aboard. Likewise, the COB is not necessarily replaced when a more senior sailor reports aboard. The selection process for a COB starts with the commanding officer's recommendation, which goes to the Naval Military Personnel Command via the Squadron Commander. Nominees are interviewed by other COBs. When a new enlisted sailor joins a boat's crew, the COB is usually one of the first people the new sailor will meet. Although the role of COB is outside the direct Chain of Command, the COB has tremendous authority and responsibility as top enlisted sailor of the submarine

MHCM A Master Chief of some rating I'm not familiar with (I've been out since 1985 and there have been many ratings combined with other ones to make NEW ratings. MH must be a really new one, unless you mean HM which is a Hospital Corpsman. Unlikely to have an HMCM on board a sub.)

XO is the Executive Officer and his Rank was Commander.

FTCM is Fire Control Technician Master Chief (E9)

MHCM there is not a Rating or was for MH there is HM which is Hospital Corpsman (Medic) or MT which is Missile Technician or MM whichi s Machinist Mate that could be on a Submarine

COB is the Chief of the Boat who would be the Sr Enlisted on the Sub.

Your right about XO

COB means Chief of the Boat, usually the highest ranking Chief Petty Officer. It corresponds to Command Master Chief in the Surface and Shore units

FTCM is Master Chief Fire Control Technician

I'm not sure what a MHCM is but he's a Master Chief Petty Officer too

To complete the above answers, PCOB is the Prospective Chief of the Boat. He has already reported aboard and is in the process of relieving the current COB. I think MHCM should be HMCM.