Unfortunately the map item you pick up actually isn't super helpful, since it doesn't mark the points on your Pipboy. This map shows Eliza's drawing indicating the location of each generator that has to be powered on to access her family' hideaway. On one of the metal racks at the top of the building you will find Eliza's Map Of Home. Head to the destroyed barn building marked with the “Vim Quartz” poster and head upstairs. Have your weapon ready, as the way there is filled with insane trappers and the docks themselves are overflowing with vicious wolves.
The map marker location you are looking for is the Cranberry Island Docks. To begin your quest to find all the goodies, head down to the smaller island to the south of the main Far Harbor land mass.
This mini-quest seems to break how Fallout's ghoul system works for the sake of an interesting side story, indicating that ghoulism can apparently be transferred like a zombie virus, which has not been the case anywhere else in the series. In another nod to the DLC's horror inspiration, there's also a little secret area where a family kept a large stash of equipment in case of the apocalypse. There's the hard-to-find vault 118 with its amazing Brain Dead quest, the series of hidden Islander's Almanac magazines, a secret quest to tackle the massive Shipbreaker monster, and even some new perks to be found.