Along with getting to the island, the mod also allows you to have  more immersive experiences  on the island (there is no sniper). Just head to the map blip SS. Guarma in St. Denis to begin your journey.

Once on the island, your horse will always be there (may not heed your whistle but it's there in "spirit") no matter if you die or save and reload the game. It's there but all the components might not be (but you'll have a basic saddle load out for your trusty steed nonetheless). So will the captain of SS. Guarma. He'll be fishing away at the dock waiting to return you back to St. Denis at your behest (you paid good money!). Stay as long you wish. An hour, a day, a week, forever? Your choice. Don't try to shoot the captain though, it's bad manners and he'll probably just ignore you anyways. All that sailing at seas and drinking Guarma rum gave him deaf ears and a steel body. 

Now that you're on the island with your horse, feel free to camp any where. Just go to a place you like and go into the resting scenario. There you will find a prompt to create a camp (Tip: if you want your camp pointed at the water turn your back to the water and go into resting scenario then click create camp). Once you have your camp created, you can cook on your grill (seasoned meat included), sleep, take a piss, play the guitar, smoke cigarettes, and even drink (yes you can get drunk). Careful though, if you drink too much, you'll probably pass out. If that happens, either sleep it off in the tent, use some relaxing scenarios to slowly wear off the effect, or just go about your day slowly recovering from your massive hangover.

On your journey around the island, be sure to check out the beach around the port. Word on the island is that sometimes a drunken fisherman leaves his canoe unattended. You can "borrow" that and see the island in a different perspective on the water. Careful though, those waves be unpredictable and you might scuttle your boat if you drift too far out so follow the shore line and enjoy your stay.

Special thanks:

• To all of the guys and gals on the RDR Modding Discord server for native code help, feedback, and ideas:  https://discord.com/channels/981043217350938714/
  

Installation:

• Just put ImmersiveGuarma.asi in your game's root directory.
  

Bugs (that aren't actually bugs):

• The script expects you to use the mod as you should (ie. travel to and from Guarma using the script). Do not try to use this script and teleport to and from Guarma using a trainer or other methods. It will screw with the script causing "bugs". It knows when you are on Guarma or not and acts accordingly. It's necessary. 
• Going to Guarma without recognizing your horse will leave you without a horse on Guarma. This is because of the game and how it spawns horses. Whistle for your horse before using the script. If the game says "horse out of range" find it, whistle for it, then travel.
• When respawning.loading on Guarma, and you want to travel back... Your temp horse will not come. It will no longer exist once you travel back. Your main horse will always. When you arrive on the docks in St. Denis coming back from Guarma by just yourself, whistle. If you get a "horse out of range" go out to the road and your original horse will come to you. 
  

I have no control over these above. Blame the game developers!
Known Actual Issues:

• Horse will not react to your whistle while on the island.
• Custom horse loadouts (saddle, bags, blankets, bedroll, etc etc) may or may not load on your temporary island horse after a new reboot of the game.