I have given it a lot of thought for the RCM (Royal Canadian Marines)
The POD might have to go back to the war of 1812. During the Great Lakes battles, the Royal Navy train up a group of Canadians to help supplement the Royal Marines. The battalion gets battle honours.
Fast Forward to Boer War, when building up, the same battalion gets fully funded and is used as the first group to head south, gaining more battle hounors.
During WW1, the battalion becomes a full brigade and is used to assist in any landings. During the war, all ships carry at least 1 squad of this regiment due to their special training in ship to ship and navel landings. One of the squads capture a U-boat and they become heroes for doing it.
Before the war ends, the battalion become the RCM.
During the interwar years they train with both the Royal Marines and the USMC to create their own blend of Marines.
During WW2 and the Dieppe Raid, they show what needs to be done. The raid is preplanned with them and is a success. They keep it up with Juno beach and Sicily. If a beach needs to be stormed they can do it, if a ship needs to be taken they can do it as well.
After the war, they are made in to 3 Brigades, 1 east coast, 1 west cost, 1 Québec/Francophone.
They have a very friendly rivalry with the USMC and the Royal Marines,
On the West Coast their main base is up in Comox and there is one of the greatest obstacle courses known. Every year teams from the RCM, RAM, RM and USMC all compete to finish it with the best times.