Nina, Ryu and Bo will need to head here shortly before entering Agua. At the bottom of the cave there is Cleansing Water, which will remove the zombies from Romero. Once this has been achieved, the party will receive the Tablet needed to enter Agua.
Before entering Ease Cave, make sure you have spoken to the old lady near the graves at night. Her husband will allow you to take the Water Jar from below his grave, which you will need to pick up the Cleansing Water.
From the entrance go north and take the first right turn, to find a treasure chest containing a Herb. Head back to where you were, and take the left turn. When the path splits, go right for a chest containing another Herb. Then take the left split. As the path turns, you'll come across a chest containing a NiceHT, which should be equipped on Nina. Follow the path around, and take the next right turn. Follow the path, past the stairs, until you come to a chest containing an F.Stn. Then go back to the stairs, and go down.
Take the south-eastern path first, for another chest containing a herb. Then backtrack to the stairs, and go north. Take the first right path for a T.Drop, then backtrack and keep heading north. Follow the path around till you reach the stairs, then go down to the next floor.
Take the north-west path to a set of stairs heading up, and pick up the last two chests from the previous floor - a BronzHT and an Apple. Then back track and take the south-western path.
Head downstairs, and walk into the water (you won't get very far). Press A and the Water Jar (received in Romero) will fill with Cleansing Water. After receiving this, the party will automatically head back to Romero and sprinkle the water over the graves.