Short answer: In Fallout 4, you collect Dirty Water by filling empty bottles at natural water sources (rivers, ponds, or water pumps in Survival mode can convert bottles to Dirty Water). You’ll need empty bottles in your inventory to collect it. Details and tips

• Where to find Dirty Water
  • Natural sources: any rivers, ponds, or lakes you come across. Approach the water and use the action prompt to fill your bottle(s) or grab Dirty Water depending on your mode.
  • Beer bottles or other empty containers: you can collect with any empty bottle you carry. Common places to stock up on empties include breweries, bars, or vendors that sell bottles.
• What happens in Survival mode
  • Filling an empty bottle at a water source typically yields Dirty Water.
  • If you fill a bottle at a pump, you may get Purified Water instead, so target natural water sources when you want Dirty Water.
• Practical tips
  • Always carry multiple empty bottles so you can collect more Dirty Water on the go.
  • If you’re short on Dirty Water for cooking or certain chems, you can visit settlements or beverage vendors to buy it, or build beverage stands in settlements to stockpile it locally.
  • If you want to bypass farming, you can also obtain Dirty Water from vendors or from certain containers in buildings, though sources vary by location.
• Common issues and workarounds
  • If you run out of empty bottles, scavenging for containers or purchasing empties from vendors is helpful.
  • Some players find that repeated collecting from the same water source can yield multiple instances of Dirty Water, so you can plan a quick route through a mapped area with good water sources.

If you’d like, specify your current playthrough details (difficulty, Survival mode on/off, and whether you’re near any water sources), and I can tailor a quick route to gather Dirty Water efficiently.