For those who have played the story and getting a bit bored then try this:
I finally found the glitch to get back to Guarma island, I've already posted photo's of my 'visits' to Mexico which has no animals or anything. So to get back to Guarma, like crossing to Mexico, you need to have completed the story.
So swim across to Mexico with your Horse at a suitable place where there are no high banks so you can actually get your Horse onto the terrain. It's best to cross down near Amarillo as there are no massive canyons to get out of and then head Nth West
You really need the Horse energy cheat via Wemod or similar and then you can gallop endlessly till you are completely off the detailed part of the map, the vegetation is now gone and you can see grid lines in the terrain, just keep going as far away from the river as you can.
Eventually you see what looks like a lake so head towards it and suddenly with no warning you drop off the end of the map, you end up under water & die, you then respawn on the island.
Everything is alive, animals, parrots, iguanas etc. there are people in the nearby huts who make comments as you pass, I headed for the fort & a hidden sniper killed me, I spawned again and approached from another side, the soldiers ignore you unless you shoot first and then all hell breaks loose. I guess it's possible to wipe them all out but they seem to keep on coming, I found a place upstairs in a building so I could mow them down as they climbed the ladder.
Before you plunge off the map make sure you have your rifles on your back and not on the Horse as otherwise you will only have pistols. You cannot spawn a Horse via a cheat either.
You cannot make or use a campfire as a save point but you can save as normal, when you have had enough you have to load a previous save to get back to the USA. Loading the save you made on the island works fine if you want to return, you can hunt all the animals you missed during the story, strangely you cannot pick or use any of the abundant flowers anywhere. There are the deadly Fer-De-Lance snakes in abundance and various exotic birds not found in the rest of the game.
Photo's are desktop screen capture as the in game photo's are a pain to get to work