Start by the boat the survivors are working on. Turn left and leap over to the wooden pier in the south, pictured above, then make your way towards the galleon in the distance. You're going to have to do some more jumping, but not too difficult.
When you get to the larger platform, find a ladder and go up. Scramble up the higher ledge, then grab onto and slide down the zip-line.
Once Lara lands, leap over to a horizontal beam. Bounce off it and press the action button to use the climbing axe. Move right, jumping when needed, until you reach a small hole in the galleon's hull (reward: 100 XP).