After you unlock Florida by synchronizing with the viewpoint, fast travel there and you should easily spot 2 ships, an English and a Spanish vessel. They have a random amount of loot on board. Climb slowly on board the Spanish one and loot as desired until you are forced to jump overboard. You have around 45 seconds to do so. Then head to the other ship, where you should have around a minute. Once the two ships have both sunk, open your map and fast travel to the viewpoint again. Both ships should reappear, allowing to loot them all over again. You can repeat the process as often as you like. It's a great way to find the resources to upgrade your Jackdaw.