Raft

1. Stop the raft
2. Throw some items one by one (no stack, for efficiency) into the sea at the same position
3. Full your inventory
4. Use the hook to hook the piled items, and they will drop back to the sea (because your bag is full), and hook over and over again

and here is a video: https://www.bilibili.com/video/BV1TU4y1B7SH

1. Play through the Tangaroa map until you reach the top of the central tower.
2. Punch in "4813" into the keypad.
3. Profit.