The Legend Of Zelda: Tears of the Kingdom will give you a variety of quests to always keep you busy. If you're a perfectionist, then you're striving to complete every aspect of this game which would easily make the game go from around 50 hours to 190 hours.
Some quests you find will be pretty lengthy while others you can get done within minutes. Luckily for you, getting the Pirate Manuscript shouldn't take too long. Once you're finished with this guide, you'll be one quest and one step closer to the 100% completion.
Getting Started At Cephla Lake Cave
You will receive a riddle that says, "A forgotten pirate cavern lurks at the foot of Cape Cales, overlooking the Necluda Sea. The short, shrill song of wind through lips will open the way to my treasure." To do this side quest, you should be located in the southeast portion of Necluda. Your first step is to complete one of Misko's side quests titled Cave of Chests. You will pay 100 rupees to Domidak and Prissen who will be standing right outside the Cephla Lake Cave.
Have extra stamina or build a boat due to the difficulty of swimming in the cave.
Cape Cale's Cliffbase
Once you accept the quest, you will head to the Cape Cales Cliffbase Cave located slightly Northeast of the Cape Cales on the map in Necluda Sea. Jump off the cliff using your glider occasionally, so you can control which direction you're falling. Stay closer to the rock wall, so you can float into the cave when you get to it. The cave will appear after you've been falling for a couple of seconds. Use your glider to glide into the entrance and onto this wooden platform in the water.
Unlock The Cave
In order to unlock the cave, you will have to whistle. Once you whistle, the stone door will slide to the right opening the passage for you to enter.
The Treasure
Swim to the next wooden platform in the water to regain your stamina if you are swimming. Standing on this platform, go to the upper corner of the wood facing straight ahead then, if swimming, jump off to travel straight ahead from that point. If you have a boat that you crafted, use that to travel to the next part of this mission. After a few seconds you will come to another wooden platform next to an old wooden pirate ship that you can climb on and regenerate your stamina. Using Ascend, travel straight up through the grated area of the ship.
Once up here, you will encounter enemy skeletons. If you want to avoid them, climb up to your right, onto the ledge of the ship, look towards the other end of the ship and using your glider, glide down to the barrels across from you. Behind these crates and barrels will be a chest. From this chest you will receive Tingle's Tights. If you hadn't defeated the enemies yet, the trick to defeating the skeleton ones is to destroy their heads. Be aware the getting hit by the lightning arrows makes you drop your weapons, so it's best to take out the archer first.