Upgrading Jackdaw and Accessing Captain's Quarters

Right now i am in Nassau, I haven't done any main missions here yet though, just explored got all the loot and viewpoints, and I've spent some time sailing around with the Jackdaw blowing up and boarding ships. But when i go to a harbourmaster he won't trade
with me and says something like, "I only deal with ship captains.." And when I am on the Jackdaw there is no option to enter the captain's quarters. I walk up to the doors just under the wheel, and there is no way to interact, or on any of the trapdoors on
deck leading below. Up to this point i assumed I was captain of the Jackdaw, do I have to progress further before I can upgrade the Jackdaw and enter the captain's quarters?

:)

 

Legendary Ship Achievment

It's taking awhile to get the cheat. I have to get 70 abstergo challenges. Not sure if it's faster then upgrading the jackdaw but it's probably not so monotonous and just attacking and plundering ships for cargo

 

Collectibles

Mass Effect 2 got it right. There were 11 model ships that you could collect that would then be visible in your Captain's Quarters. For the more immersive gamer, they had thousands of codex entries to collect.

 

I believe you have to complete a few missions from Nassau before you can upgrade

 

^^^ This.... you need to get to memory 3 then you can really upgrade.