Rock Spire Isle
Rock Spire Isle is one of the isles of the Great Sea in The Wind Waker. It is a long slender island with the path to a Secret Cave being blocked by rocks. To destroy the rocks, Link needs a couple of Bombs and good timing. At the end of the path there is a grotto and, inside, Link must use a Boko Stick to light up the room and destroy all the Keese. Once the last one is destroyed, a Treasure Chest will appear containing 200 Rupees.
There is a somewhat different kind of Beedle shop around the isle called the "Masked Beedle" Shop. Link needs a wallet upgrade in order to buy most of the items Beedle sells here, which include a Piece of Heart (950 Rupees), Treasure Chart #4 (900 Rupees), and a Bottle (500 Rupees).
Two other Pieces of Heart can be obtained here. One of them is found by taking down two armed boats floating around the isles: one of them will drop 100 Rupees, and the other one will leave a Piece of Heart. The second one is found at the bottom of the sea by using Treasure Chart #2 from Windfall Island.
Fishman's hint on Rock Spire Isle:
I wonder how long it's been since the light in Windfall's lighthouse went dark... It was quite a grand sight, I tell you, the way that ray of light shone out like a shaft of gold through the cold darkness. I used to gaze at it with dear Gillian... Anyway, I bet if someone could shoot a powerful spark into the top of that lighthouse, it would light up again! Of course, that's just my intuition talking... Then again, you should never doubt a fish's intuition, small fry!