I got the angler key and I used it on the key hole. It stopped the water fall and has exposed a temple entrance on the other side of the water. I don’t see how you get over there though. There is a stairs nearby but when i go down, the way is blocked with a rock that has a hole in front of it. I can see a stairs right next to the entrance to the temple. I am assuming this is how I get over but i dont know where to find the tunnel that will get me there in the first place.
The stairs at the other side near the entrance is actually the exit. This is how you get back to the mainland after you have completed the temple. To get over you need to head up to the mountains. You may have been here already and been unable to walk past due to the water running and blocking the way. In the cliff edge above you can drop down to the temple entrance below. It is a bit of a walk to get up to the mountains but this is how you get down to it.