The ships that you are seeing are NPC ships. If you hail a player it will show their rank like "Ensign Bolitho" or "Commodore Leviathan" whereas an NPC won't show a rank when you hail it. The exception is once you are in a battle instance the NPCs will have generic ranks assigned to them.
Every capital, English Harbour in this case, has NPC fleets that sit out in front for protection from ships of other nations. You can't join them while they sit there but if they would attack a passing player, or a player attacks them, you can click on the battle instance and join it to fight with them.