But there almost certainly is a reasonable justification as to why someone can get passage on a boat at this point in time.
You keep thinking that this ability is going to be used all the time, no matter where the PCs are--but that's not the case and has never been the case. Unless you, the DM, make it so that sea travel is commonly available and very useful in the first place, getting passage on a boat is not going to be particularly common no matter what the players' backgrounds are.
Plus, have you ever considered asking the player how the feature is justified? "We're on a different continent. Tell me how you expect your feature to work." Either the player will have a cool justification that you will accept, or they won't have a justification and the point is moot.