Here is where you need the 5,000 reales, to purchase the Diving Bell. Go plunder, hunt and spear your way to the mark. After that go to a Harbor Master and buy the Diving Bell.
If you have a lot of Rum and Sugar on the Jackdaw, now is a great time to sell it off. Pickpocketing the total would take way too long and poses far more risk of drawing lots of attention to you. If you have fully upgraded your rowboat and harpoons then hunting up some White Whales is a good idea as their hides are worth 2,000 a piece!
Buy it and you are ready for the next objective. Fast Travel to either Florida or Dry Tortuga to vastly cut down on the travel time.