There are currently 11 of these commands that fall under the authority of the Joint of Chief of Staffs, and even more broadly under the authority of the DoD
Unified Combatant Command
A unit of length for measuring the depth of water, and is equal to 6 feet (approx 2 meters)
Fathom
Abbreviated MSC, this is a command within USTRANSCOM
Military Sealift Command
Joe Biden
The United States Navy's birthday
October 13 1775
This is a command within USTRANSCOM that is responsible for sealift transportation
Military Sealift Command
A shot has these many fathoms
Fifteen
Abbreviated SITREP, this report is used to notify operational commands of a significant event or incident.
Situational Report
The current Secretary of the Navy. He is a Naval Academy and GW grad
Carlos Del Toro
This President is considered the father of the Navy
George Washington
This command is responsible for U.S. military operations, including fighting regional conflicts and maintaining military relations with 53 African nations
USAFRICOM
This device helps measure a ship's speed through the water
Pitsword
Abbreviated POSREP, this report describes the position of a ship
Position Report
The current Chief of Naval Operations (CNO)
Michael Gilday
The oldest commissioned warship afloat in the world and one of the original six frigates.
Constitution
This command has been the main American presence in many military operations, including the Persian Gulf War's Operation Desert Storm in 1991, the War in Afghanistan, and the Iraq War from 2003 to 2011.
USCENTCOM
The side of a ship above the deck
Bulwark
Abbreviated CTF-63, its headquarters are in Naples, Italy and it comprises oilers, provision ships, and repair ships
Combat Task Force 63
Commander, Naval Education and Training Command 
Rear Admiral Garvin
On this date, the US bombed Hiroshima
August 6, 1945
This command is responsible for providing contingency planning, operations, and security cooperation for Central and South America, the Caribbean (except U.S. commonwealths, territories, and possessions), their territorial waters, and for the force protection of U.S. military resources at these locations.
USSOUTHCOM
Plimsoll Line
Known as USTRANSCOM, this unified command is the overarching command of MSC, AMC, and SDDC
United States Transportation Command
This retired four-star Army general succeeded Michael Esper.
Lloyd Austin
In this year, on this day, Pearl Harbor was bombed by the Japanese.
December 7, 1941