Walk over the collapsed mast and take cover as soon as possible. Three new guards have just shown up on the galleon, and considering the game won't allow you to get rid of them without raising alarm, resort to ambushing them to take them down before then have a chance to fight back.
When the shootout is over, make your way towards the zip-line outlined in the distance. Lara will grab onto it automatically and return to the beach. You can now rest in the nearby Survivors' Camp. The stage will come to a conclusion and Lara will receive a Compound Bow from John (reward: 400 XP).