I didn't find it too overly difficult but I still have quite a ways to go until I finish it. But before you start they give you a difficulty option, I am on the second easiest which would be considered normal difficulty but there is also an easy option. My friend had this game and after hunting with him for a couple hours I had to get it. You will get a rush hunting great whites in your boat.
You can also make the game much easier upon discovering that you can purchase body armor in any store. The stealth mechanic are very forgiving too. Crouching inside of a bush makes you invisible to enemies who are several feet away. Your biggest dangers will be: fall damage, making too much noise in the jungle and getting mauled by animals, and setting yourself on fire.
If you really get stuck, you can cheese it with special syringes that grant 20-30 seconds of damage invulnerability or 1-hit kills against enemies.
"I'd rather betray the world than let the world betray me." -Cao Cao