The Sons of the Forest rebreather is one of the first crucial pieces of gear you'll need to begin solving the island's mysteries. Luckily, it's a relatively quick one to grab, compared to the shovel location and rope gun location, that is. The rebreather is in a cave on the northern beach which should be very close by if your helicopter crashed at the beach.
Take a look at the map down below for the exact location of the cave entrance. You can also use this interactive sons of the Forest map to help locate it. Here's what you need to know to navigate it and snag the rebreather inside:
How to get the rebreather in Sons of the Forest
Like other caves, this one is dark inside so make sure to grab a torch or lighter. There are enemies waiting in the dark, but the path to the rebreather is the only one so far I've felt I could actually sprint past them all without getting killed in the attempt.
Head forward through the cave until you hop down to a standing pool of water with an innertube floating in it. Continue on past to the next room and keep an eye out for one of those bright work lights illuminating a path to the right. Don't follow it, and take the left fork in the dark instead, even though it's quite tough to see. You'll need to juke your way past several of those slimy cave enemies in this same room. Check the images down below for a visual reference since you won't have time to stop and scratch your head about it.
That's the biggest diversion you need to watch out for, because the right side path has a lot more enemies. It does lead to other important pieces of gear like a stun gun which you'll want to pick up at some point. If you just want to snatch and grab the rebreather though, continue on past that fork and just keep pushing forward through each cavernous room you reach. When you're looking down on a large pool of water with a big pool float and work light on the other side, you've reached the place.
Don't fall in the water until you've got that rebreather equipped, because there's a shark down there waiting to munch on you. Instead, skirt the right side of the room to reach the other side and find that handy rebreather waiting for you.
Throw your new gear on and you can escape by diving down into the water and swimming through a hole in the cave wall beneath you that eventually leads back out to the ocean.