There a house that is "haunted" but in reality is the lair of smugglers using it as a depot for the town of Saltmarsh. The climax of the adventures comes as the adventurers are hired to turn the tables on the smugglers and hit their ship the Sea Ghost.
In my campaign I adapted this module in several ways. First is the house itself; while "haunted" by the smugglers, it was once the home of a sinister alchemist/magician. If some plot in your campaign involves magical lore or a old magic-users this may serve as the perfect place to plant the initial seeds of this portion of your plot.
This module leads into U2 Danger at Dunwater and U3 the Final Enemy which climax with a fight against the Sahaugins. If you decide not to go with that plot the smugglers can serve as the initial plot seed for a larger criminal organization that becomes the nemesis (or allies as the case may be) of the PCs. In my own Majestic Wilderlands it has served as an initial lead into the Slaver's network and as a lead into a plot involving the Skandian Vikings.
Finally the Sea Ghost portion of the module serves as a useful template for a sea-faring ship. I used this as template in a number of campaigns where the player owned their own ship. It it is one of the few sources for a complete deckplan of a sailing ship.