Spelljammer-ish campaign idea

So, the setup: the players are at a tavern in a worldlet resembling a sphere of ocean water floating in space with tropical islands dotted across its surface, or bands of water floating in space with the islands floating on them. They hear a commotion outside at the docks, and someone bursts in and says that a famous pirate ship has returned. 

At the docks, the wrecked form of a ship flying a ragged pirate flag drifts in the distance, slowly limping towards the dock. The dockmaster sends out a tug to pull it over. The ship appears derelict and abandoned, but as the tug workers begin to affix ropes to it, a heavily injured man crawls up from bellowdecks. As the wreck is pulled back, those assembled at the dock can hear the man raving about how he's the only one left and that someone or something is chasing him and is almost there. 

As the man is carried back ashore by the tug workers, the island's bell tower rings three times, a signal that its scrying team has detected hostile vessels inbound. The crowd at the deck sees forms of ships creating the horizon and approaching. Dockworkers rush to man the fort's cannons, the braver captains sail out to meet the aggressors, and the more cautious ones prepare to set sail in the opposite direction. 

What happened was that the famous pirate sailed deep into the territory of a feudal theocratic empire and gathered a massive amount of treasure, but was brought down and forced to abandon his bounty on a remote Arrakis-like planet in their territory. One of his ships managed to escape and make it back to the port, with the man aboard being the sole survivor. The empire sent ships after him, and the first major action scene has the protagonists/PCs help fight off the attack. The story then has them journey into the empire's territory to avenge the fallen pirate and reclaim his treasure. 

So it's basically "Space One Piece vs. Dune"