After you get Misko's treasures from Cephla Lake Cave, you can pay Domidak and Prissen to learn about the pirate treasure.
Quest Location
If you have completed Misko's Cave of Chests and Misko's Treasure: The Fierce Deity, you can talk to Domidak and Prissen outside of Cephla Lake Cave and pay 100 rupees to learn about the pirate treasure.
How to Complete the Quest
Go south from the Hateno Ancient Tech Lab until you reach the ocean. Make a raft using the items on the shore: a wooden plank, a fan, a battery, and a steering stick. Go to Cape Cales Cliffbase Cave. Domidak and Prissen gave you the clue to go farther into this cave:
Misko's treasure platform is to the left, but it's empty. However, there is a wooden plank under the bow of the ship. You can stand on this plank and use Ascend to reach the ship. Defeat the Stalkoblins and Stalmoblin, then look behind the wooden crates near the cobwebs to find a treasure chest. Open it to get Misko's treasure, Tingle's Tights.