Water Is Life...Or Death, If You're Not Careful
Water 'droplets' you find while scavenging natural locations (forests, hills, empty fields, etc) are NOT safe to drink unless you test them somehow first. You can get infections (and die) or be posioned (and die) from water that is contaminated. Boiling water to sterilize it will kill infections, but it won't remove poisons (like you find in the Great Black Swamp). Likewise, water purification tablets will kill infections, but they don't remove posions either.
Bottles of water you find on corpses (including people you just defeated in combat) are NOT safe to drink unless they contain water labeled as 'water (sterilized)'. this is very important to remember - just because some guy was carrying bottles of water, it doesn't mean he got it from a safe water source. Never drink any water you find on a cultist (they wear Blue Sashes, or have them lying on the ground next to them if you find one already killed by something else). Cultists have diseases, and everything they touch/use is likely to be contaminated.
Water from a marsh is almost always unsafe to drink. Water from a river is more likely to be safe to drink, but it is still risky.
if you are lucky enough to have a battery powered Water Tester (and charged batteries) you can check any water droplet you find to be sure it has no infections or poisons. These items are pretty uncommon, though, and they take up space/weight that you may not want to give up.
There is an easier, faster way to check for safe water.
Never Drink Your Last Bottle Of Bottled Water
The way the game currently works (Jan 2015), only exactly identical items will stack, if the items are able to stack at all. This game mechanic can be exploited to help you identify water that is safe to drink, without having to boil/sterilize/purify it first.
- one bottle of sterilized water that you know is safe to drink (you bought it from a vendor, or you found it while scavenging an urban location, not something you found on a dead body)
- as many empty bottles as you want to carry your drinking water (usually 3-4)
- a natural water resource (a hex containing a river, lake, stream, or marsh)
We are going to call this bottle of guaranteed safe water your Test Bottle. Never drink this, never accidentally use it up while boiling rags, never drop it or sell it. We will call the two water droplets inside of it your Test Droplets. Be careful that you never use these two droplets while crafting/boiling rags, drinking water to satisfy your thirst, or destroying unsafe water.
- open the screen that shows the items on the ground at your location, and your character's carried/worn gear
- take the water droplets out of your Test Bottle (select the bottle, right click and choose Empty Out).
- move that stack of 2 sterilized water droplets down to the bottom of the screen, away from everything else so you don't confuse them later.
- unstack the sterilized water droplets (hold Shift key while left click-and-drag them), put them side by side.
- If you don't already have untested water droplets on the ground, select the natural resource (river/lake/stream/marsh icon) and Use it (number 2 key) a couple of times to produce a bunch of water droplets in stacks. Now pick a stack of 2 untested water droplets, drag it down near your test water droplets.
- Unstack the unidentified water droplets, then pick one up and try to drop it onto one of your test water droplets.
- If it stacks, the water is safe to drink. Unstack it, then move it to one of your empty water containers. Note: It won't be labeled 'water (sterilized)' but it can't stack with your good water unless it is identical, so you're good to go. Be careful to put the right droplet of water in your empty container, leave the droplets from the Test Bottle where they are for now.
- If the water droplet does NOT stack, it is not safe to use. You'll end up holding your good water droplet, so just drop it right back where it was and you'll swap back to the bad water droplet. Move the bad water off to one side, make sure you still have your two good Test Droplets, then destroy the bad water droplet (number 3 key). This helps to avoid deadly mistakes later, trust me. Again, make sure you destroy the bad (not stacking) droplet, not your good Test Droplets.
- Repeat the process until you have filled all your empty water bottles with safe water, and have completely filled your thirst bar by drinking safe water.
- Stack your two Test Droplets again, then replace them into your Test Bottle and put it safely away.
This method is a little tedious, with the dragging and clicking and such...but you never get posioned or infected from your water sources, and you don't have to carry any special gear (or make a fire) to do it. That's the trade off for this exploit - your time in real life making sure your water supply is safe in game.