I always love the classic twist of "The person who hired the team is actually the baddie committing insurance fraud" plot, and I feel like this one could work.
Perhaps there is a merchant, who has recently had a couple of ships lost to piracy. The merchant is a savvy entrepreneur however, and has always made sure to purchase large insurance contracts on their ships, using these pay-outs to upgrade to bigger and better ships each time.
The insurance provider is also savvy though! So here for the merchant's fourth voyage, on their biggest, and most expensive ship yet the insurance company has insisted that the merchant hire extra protection for the ship. This is where the PCs come in, being hired to ride aboard the vessel and protect it over it's journey, with a promise of substantial payment on it's return.
Here's the fun part. The merchant has been loading each ship with a few crates full of goblin minions, who wait for the second or third day at sea to come out at night, and kill the crew, and take the ships to a secret hiding location, where the ships are being repainted, renamed, and prepared for resale.
You get to have some classic who-dunnit fun here. You can plant seeds that perhaps the merchant is shady. Perhaps one of the crew members is shady. Maybe there's one crew member who survived the last attack and was rehired for the current voyage. He claims the ship was attacked by sea dwelling demons who somehow must have been able to magically teleport themselves aboard the ship and into the hull. He didn't get a good look at them before being thrown overboard but has no other reasonable explanation other than something supernatural.
When the players uncover the plot they get lots of potential options. Maybe once they've been bested the goblins will try to bargain with the PCs. Offer them treasure and a cut of the payment. Perhaps they themselves were planning to double-cross the merchant and take the ships to start their own, goblin-pirate fleet. They could offer the PCs a position in the fleet and command of one the ships. Maybe if they discover the location of the secret island and the other ships, upon reporting this to the insurance company the company agrees to reward them by gifting them the smallest of the ships. Maybe the PCs have a bit of pirate in them themselves and decide to take the ships on their own, becoming a pirate gang.
Lots of room to seed hooks for future sea-faring adventures.