You have to continue the quests, look at your notes, you have to do the quests for Silas (in the map of the boats), and then he will give you a way to go in the Depot A.
But you'll see it's a really difficul map, with mutants... So you should keep on with Silas and other pnj's quests and when Silas quests are over, and almost all other, you'll be able to really inspect Depot A.
But don't hesitate to try to go in there once you know the way, because there are a lot of ennemis and cleaning up the path will help you gaining experience.
