The Hold is a sea grotto, a cave in use by humans. It is Day Mountain Sea Cave to be exact. It consists of passages and chambers. The grotto has an outlet to the sea by way of water. People have to dive into the sea and swim out. During the tide, the sea access chamber pretty much fills up with water.
Sea Access Chamber (room 1): this is where the PCs start. The Sea Access chamber is the lowest chamber of the grotto and fills up with sea water every two hours due to high tide. During low tide, the sea retreats.
Pirate Stash (room 2): Contains the first booty that the pirates stash. This treasure includes plate armor and lesser armors (leather jerkins and chain shirts) along with a fresh change of clothes. It also has weapons (quarterstaves, bows, swords, shields, arrows). With the exception of a masterwork longsword, the weapons are of standard quality.
Kobold Chamber (room 3): Kobolds are a part of the crew, but the racism of the pirates has forced the kobolds to act in a henchman like fashion. The four kobolds are alerted to the presence of the PCs and attack to defend the loot that the PCs had already taken.
If the PCs take any of the kobolds prisoner, they can interrogate the kobolds to find out what or who is in the grotto. If pressed, the kobold will reveal that he works for Calypso Jack. Calypso Jack is the most notorious pirate in these waters.
The passage leads to an upper slave chamber and to a second pirate stash chamber.
Slave Chamber (room 3a) -- several slaves are located in this chamber. They are made up of human and lizard-folk slaves. They can only be freed with a key that the pirate conjurer in room 3b has.
2nd Pirate Stash Chamber (room 4) -- guarded by a conjurer. The pirate conjurer will conjure two monsters to fight for him and use offensive spells (magic missile, lesser firebolt, sleep) in order to overcome the PCs.
The stash includes alchemical apparati and rare spell components. The chamber was going to be set up as an alchemical lab. The conjurer does have the key to free the slaves in the slave chamber.
Bat Chamber (room 5): This chamber contains bats and bat guano. If the bats are disturbed, they will fly around to confuse the PCs.
Pet Chamber (room 6): Calypso keeps a Shadow Wolf as a pet in this chamber. The pet doesn't like intruders and barks loudly and attacks the PCs, alerting the other pirates.
Ship chamber (room 7): Contains the wreck of the pirate ship Chimaera. The pirates here attack en-masse governed by mook rules (4 hp total, they are either up or they are down). Here they face First Mate John, which is a powerful ogre in an outfit like the Jack of Hearts. He's armed with large hammers and can get magic hammers when he's down to 60% of his hp.
Captain's Cabin (room 8): Calypso Jack is here. SHE is wearing her best clothes and is armed with two flintlock pistols and a rapier. She has two pirate guards guarding her. In her cabin, she holds the jewel -- the young princess Ihitha of a far away kingdom.
Secret Passage to Castle Prospect (room 9): the secret passage leads to the dining room of the Castle Prospect. An implication that Calypso Jack is in league with the Count of Castle Prospect.
Passage out (room 10): another passage that leads to freedom and the coast.
Act 1: the PCs find themselves in a grotto after swimming for their lives after their ship was wrecked.
Act 2: Upon learning that the captain is holding a valuable jewel and that there is a way out besides the access chamber, the PCs should be motivated to find the way out.
Act 3: Mook facedown! The PCs face a pirate horde governed by mook rules. The fight should be fast and furious with the PCs facing down First Mate John, a "boss" like NPC.
Act 4: Calypso Jack -- the adventure reaches a crescendo with the face down of Calypso Jack. She is armed with flintlock pistols and a rapier and she isn't afraid to use them. Calypso Jack is protected by two armed pirate guards and is safeguarding the jewel -- the princess Ihithia -- in her cabin.
Denouement -- The jewel is safe, and the PCs find a way out of the caves. They even freed the slaves. Justice has been served and the authorities can take the pirates away to serve time. The PCs can continue their adventures.
Sequels! -- Adventures like this can spark sequels. Possible sequels include --
Kingdom of the Jewel -- the PCs escort Princess Ihithia back to her kingdom. The adventure to a near mythical kingdom can be a campaign unto itself. The kingdom is a Medieval or Ancient India analog.
The Count of Castle Prospect -- Perhaps the count of Castle Prospect was in league with the pirates of Day Mountain Grotto. Perhaps he was on the take. The PCs can investigate his shady past and expose the pirate for what he is -- a crooked aristocrat.
The Ghosts of Day Mountain Grotto -- perhaps, after many days, the grotto becomes haunted by those that were slain by the PCs. A new generation of PCs return to the Pirate's Hold to clear the grotto of ghosts and goblins. The PCs return to find Captain Calypso Jack's legacy and her last will and testament -- bequeathing the Chimaera to the PCs. Which they can fix up into a nice thieves' den.