Thirst Things First is a quest in Fallout 76 that starts right after First Contact.
Your task is to find Kesha McDermott and eventually investigate the water source to see if it’s contaminated.
Here’s a walkthrough of Thirst Things First in FO76.
Search for Kesha in the system (optional)
You can first check for Kesha McDermott’s location using the self-registration terminal. It’s only optional and you don’t need to do this to find her.
If you do prefer to use this, look for Kesha’s name in the terminal.
Search for Kesha at the river
Now, get to the river just nearby. The location is shown in the map below.
You should find her body at the river. Loot all of the items she had on her. A voice recording should also play immediately.
Next, get to the dirty water and drink it. You will then find out that the water is contaminated and get infected with rads.
I’m not sure if you can just collect it instead of drinking it.
Test a sample of water from Flatwood’s pumps
Go to the water pumps at Flatwood. The exact location is shown in the map below.
You will see two water pumps beside a shack. You can drink and collect the water.
There will be less rads this time, but still it lowers your HP. You have to collect it this time.
Analyze the water testing kit’s results
Get back to Kesha’s lab, which is where you were supposed to find the Overseer in Flatwood.
The exact location is shown in the map below.
Get to the diagnostic terminal and use it.
Test the sample of water you collected earlier, and obviously it’s contaminated.
Get out of the building and to a pot nearby, in the location shown in the map below.
Use the cooking station and boil the water.
Check in with the self-serve kiosk
Once you’re done, get back to the self-serve terminal / kiosk in the building.
Click on check volunteer status and you’re pretty much done.
The quest completes here and you will earn some items as rewards.