You receive this quest from Gerdy Hooke automatically after the Making Juice quest is finished.
She gives you a Water Pitcher and asks you to get her some Water to flush down the juice.
Get some water for Gerdy from the ocean.
Other items gained:
The Water Pitcher can be wielded like any other tool or weapon. Equip and use it when swimming to get Water, which is added to your inventory. Then talk to Gerdy to complete the quest.
After completing this quest you can talk to Gerdy to start the next main quest Find a Doctor.