Assassin's Creed Black Flag Resynced introduces a total of 12 Ultimate Ship Upgrade blueprints for the Jackdaw. Some of these blueprints maintain their original locations from Assassin's Creed IV: Black Flag, while others have been slightly or entirely relocated. This guide details the positions of all 12 upgrades.
Hull Armor Ultimate Plan
The Hull Armor Ultimate Plan can be found at the San Ignacio Wreck, located at coordinates 352, 744. Players will need the diving bell to access this underwater area. Navigate to the rear of the wreck to discover a large treasure chest containing the plan.
Round Shot Ultimate Plan
To acquire the Round Shot Strength Ultimate Plan, travel to the Kabah Ruins at coordinates 738, 120. The diving bell is required. Proceed to the furthest section of the ruins, taking care to avoid the shark in the vicinity, to find the treasure chest with the plan.
Mortars Ultimate Plan
This upgrade is located within the Antocha Wreck at 600, 636. Use the diving bell to reach the underwater site. Inside the wreckage, move to the right side to find the Mortars Ultimate Plan within a treasure chest.
Heavy Shot Strength Ultimate Plan
Unlike its placement in the original Black Flag, the Heavy Shot Strength Ultimate Plan does not require the diving bell. The blueprint's treasure map can be found on Ambergris Key at 30, 159. Look for an area with a broken shade sail and an opening above. Swim toward this opening and dive underwater, navigating through a long tunnel that has a midway cove for air. At the end of the tunnel, enemies will be present. The Ambergris Key map is located on the second floor. This map will then direct you to Misteriosa (270, 175). At Misteriosa's highest point, near the synchronised fast travel point, find the waterfall and dig for the treasure to the left of it.
Swivel Ultimate Plan
The Swivel Ultimate Plan is accessible via the Devil's Eye Caverns at 453, 331, requiring the diving bell. Once underwater, follow the path to the right and north, as the left path serves only as an exit. The correct route leads to a dry land area where a treasure chest contains the Swivel Ultimate Plan.
Ram Ultimate Plan
This upgrade is found at the La Concepcion Wreck (150, 272), necessitating the use of the diving bell. Underwater, proceed directly north from the diving bell's location to find a treasure chest holding the Ram Ultimate Plan.
Fire Barrel Strength Ultimate Plan
Locating the Fire Barrel Strength Ultimate Plan begins with finding its treasure map on Santanillas at the northeast corner (191, 218), situated near a skeleton. This map guides players to San Juan (454, 462). In San Juan, the diving bell is not required. Approach a small boat situated offshore and dive down to find a tunnel opening on the seabed. Swim through this tunnel, which will lead to an area with enemies. The treasure dig spot is near the first enemy, by a ramp.
Heavy Shot Storage Ultimate Plan
The treasure map for the Heavy Shot Storage Ultimate Plan is on Cayman Sound island at coordinates 298, 311, found on some wreckage on the upper north side. 
Fire Barrel Storage Ultimate Plan
This upgrade's location differs significantly from the original Black Flag and is more straightforward in Resynced. The treasure map is found at Petite Caverne (864, 237) next to a skeleton. 
Broadside Cannons Number Ultimate Plan
This plan is situated in the Blue Hole at coordinates 437, 145. Players must use the diving bell and, once underwater, swim to the location indicated to find the treasure chest containing the Broadside Cannons Number Ultimate Plan.
Mortar Storage Ultimate Plan
The treasure map for the Mortar Storage Ultimate Plan is located in Tortuga (852, 352) on the northeast side, buried near a skeleton. The map leads to Matanzas (304, 622). Navigate through the village to the back of the island and proceed through a waterfall; the treasure will be found behind it.
Harpoon Strength Ultimate Plan
The map for the Harpoon Strength Ultimate Plan is found on Mariguana Island (847, 514). Players need to crawl through an opening, and at the end, a skeleton will have the map in front of it. The treasure is located at the southernmost point of Andreas Island (545, 693), situated behind two trees.