The descent to the cave is located to the left of the garden house (in the picture there is Liza there). Go down the path to the cave under the house.
After you watch the cutscene, jump into water and dive in the area marked orange on your mini-map. The part that you are after can be found next to the source of light.
The head that you have found needs to be taken to Daisy, who can be found aboard the ship. After watching another cutscene, approach Liza near the fire. After you talk to her, leave the cave, which will finish the task.