Drive a boat to this location and a glitch will occure; your boat will suddenly fall to the bottom of the ocean. You will literally see no water with fish floating in the air. There should be at least one jellyfish nearby. I found that throwing a molotov will get you an entry.
The screams through the night. The hot air of explosions shaking the treetops. The bodies. Some faceless. They were only following orders.
Happy hunting! =)
Source of info and tested: http://ca.ign.com/wikis/far-cry-3/Jelly_Fish
I was skeptical at first as well, but saw someone on the source link I provided had posted the info. I tested it and worked.
The only drawback would be that once your at the bottom of the ocean floor, jelly fish are sporatic when they spawn, so you might have to wait until they float near or spawn near.
The only way I found to kill them is when they are nearby and floating along the sea floor so that a molotov would hit it. If they're floating higher up, I haven't found a way to kill them that way yet.
Killing a jelly fish is based on luck depending where they spawned.
The first time i went there, there was a gold jelly fish laying on the seabed so i was able to burn it with molotov. I checked the handbook and in wildlife the jellyfish was in there, the description for it was wierd though.
But then i went exploring the ocean floor and went to the edge of the dry part and started swimming again. i swam to the top of the ocean and then tried swimming back to the dry area. When i reached there i literally fell down to the ocean floor and died.
When i respawned i checked the handbook and the jellyfish didnt save. I went back to the dry spot again but this time there were no gold jelly fish on the floor. Just the white ones floating around but they were out of reach of the molotov so i couldn't kill them.
I left to get on with the game and havent been back.