Hena's Fishing Hole - Piece of Heart Location
After clearing the Twilight out of the Lanayru Province you'll be able to access Hena's Fishing Hole at any time. This place is found in Upper Zora's River just north of the place you help out Iza for the optional Bomb Bag as well as the Bomb Carrying Capacity increase.
The Fishing Hole is pretty easy to miss since it's nothing more than a door (with a fish hanging on it) in the side of a cliff with a sign out front. If you need help finding it take a look at my screen shot below. I am standing just outside the entrance.
Within the Fishing Hole there's only one building, inside you'll find Hena whom will offer you a canoe and a chance to fish within the Fishing Hole for 20 Rupees. Accept her offer as you'll need the canoe to grab this piece of heart.
Once you get in the canoe paddle out to where the piece of heart is, if you need help finding it just take a look at my screen shots. It's not very far away from where you start in the canoe at. You'll want to cast your line at the piece of heart to grab it off of the cliff - it's as simple as that.
Before leaving this Fishing Hole you may want to spend a little bit of time Fishing here (not in the canoe) for the Third Empty Bottle. For more information on how to acquire it head on over to that guide by clicking the link.
(I had to include the last screen shot cause I thought it was cute)