The first of the sidequests is the only one that you are required to partake in. You'll probably notice the many enemy ships surrounding the island. Destroy all of them to reap a multitude of rewards, though the one you should focus on is the golden one - after destroying it and claiming the treasure that was inside you'll come in possession of the Triforce Chart #5. The other ships that you destroyed will give you an Orange Rupee and a Purple Rupee.