In the N64 version you used to be able to get the fire arrows before completing the water temple. Shoot the sun, spawn a scarecrow at the drop spot, hookshot to it. Now in the 3DS version I'm having an issue where the arrows simply won't spawn when I shoot the sun. I'm currently just past the forest temple, and have gone into the water temple but didn't do anything inside yet.
They may have changed it so you need to beat the Water Temple first.
Frankly, this was news to me. I always thought you had to have the Water Medallion to access the Fishing Pond and the Fire Arrows as an adult.
They have certainly made how to get the Fire Arrows far more prominent, though. In the original, you just had the stone slab. Now, they gave you a hint in Impa's house and the actual islet where you get the arrows has a drawing on the pillars.
I got to the fishing area as an adult without even using the magic plant. I used the scarecrow's song, and then I just used a hookshot. I had to try several spots though just to find a place to hit Pierre.