The Jewel of Neptune is an ancient gem that appeared in Episode XII: Jack and the Gangsters created by the Goddess of Water and has been highly sought after by Aku for centuries, the Jewel was guarded by The Elementals who continued to guard it long after their master had disappeared from the earth. It was however eventually stolen by Samurai Jack who needed it in-order to get close to Aku. Unfortunately it was stolen by The Gangsters after they had lied to Jack about returning the Jewel to its rightful owner but instead used it to control water and sell it to people at high prices.
It is likely that the jewel returned to its rightful place after Jack destroyed the past Aku.
Hydrokinesis: Within the jewel lies in the power to control all the waters of the world.