They had been caught by the pirates and made to walk the plank. The pirates boat was anchored just off Whey-ah-Wichen.
To the pirates shock, they were able to swim, and they swam to shore. Something else that the pirates didn't know is that they had stolen some of their treasure.
They found a shaded area over looking the water with many solid large trees and bushes, allowing them to hide but still keep an eye on the pirate boat.
They decided to hide some of the treasure they had stolen at the base of Cate's Tree. The tree was strong and was as close as a tree would dare to be to the water. Inbetween the roots of the tree they laid the treasure chest, hoping one day to return.
Note: Taking the treasure chest inside the water proof case will curse you and the pirate ghosts will haunt you forever