I have heard much about forces like the Portuguese marines being established hundreds of years ago yet since modern amphibious landing tech was no where even close to invented what role did the marines have other than just elite troops or sailors with guns?
I'm most familiar with marines in the UK and US. They were formed in the 17th and 18th Century, respectively. In the age of sail marines were specialized combat troops primarily stationed on ships. One of their primary duties was to help with fighting the ship. That might mean that they performed boarding actions on other ships, either in peacetime or in a battle. Or they often were tasked with climbing into the ship's rigging and sniping at sailors and marines on the enemy ship.
They also provided security on board the ship. Essentially, the marines worked for the Captain and the officers to help ensure good order and discipline among the ship's crew. British sailors typically performed admirably, but a large portion of sailors during the Napoleonic Wars were press ganged into service, so discipline among the crew could be an issue. So guard duty would be a typical job for a marine on a ship, say to guard important cargo or provisions from sticky fingered crew, or to guard the ship's jail if it had one.
I'd also add that marines in that day did have amphibious landing technology. They were called rowboats :). Marines in that era did perform amphibious operations frequently. WW II was the great age of contested amphibious assault, but I would argue that historically that is more the exception than the rule. In the age of sail it was not such a hard thing to make an unopposed landing on a beach, away from enemy defenses, then form up and march overland to assault them from the land. Although there are also examples during the Napoleonic Wars of marines and sailors making amphibious assaults on defended positions.
On the US side, the most well known exploits of the US marines in the age of sail probably are the actions off and on the Barbary Coast in the early 1800's, immortalized in song by the US Marine Corp hymn referencing the shores of Tripoli. This involved both ship to ship and land based actions.
I'm not very knowledgeable about marine operations in the late 19th and early 20th Century, so will leave that to others. I would recommend Line Upon the Wind, by Mostert, as an excellent single book history of the Royal Navy during the Napoleonic Wars, which also does get into the use of marines during the age of sail.
I've written a couple of older answers here about sailors and Marines:
Let me know if you have follow up questions!