Sentient Weapon Walkthrough
Halfway between Bravil and Leyawiin, there's a T-shaped water inlet (look on the large map). The sign/dock is at the bottom point of the T-shape. Walk straight down the dock and keep going, right past a large tree and into an area strewn with rocks. There's a chest nestled inbetween two large rocks. This map shows the location of the chest.