In competitive multiplayer, human opponents will target your trade routes rather than attacking your heavily fortified main harbors.
Both players need their own Ubisoft Connect account and must be added as friends.
Only the host needs to own the cosmetic DLCs for everyone to see them. However, for major gameplay expansions (like The Passage or Sunken Treasures ), all players must own the DLC for those regions and mechanics to be active in the session.