The underground lake is in the Northern Forest of Runes:
- Leave Trademond
- Enter the Forest of Runes
- Go all the way right and up to the Northern Forest
- Enter the cave to the upper left
- Blow up the wall to the left
- Use a rope to go down, then another
On the Demonic Vaults quest, there's an Emerald Leaf mound to the upper right of the Vault area, next to some water.
A quick and pretty easy way get Emerald Leaf early is in the Eastern Forest (all the way to the right from the forest entrance and past a plant obstacle), though you'll have to bypass enemies or defeat/Star Knightess the tree boss.