There are typically four concerns with water sources, chemical contaminants, bacterial contaminants, taste and sediments.
Taste is separate from safety. There is some perfectly safe water that has so much sulfer or metallic flavor that it's almost undrinkable.
Sediment is usually just a problem with pipes and filters and the appearance of the water. It can make the water look dirty, when it isn't. In my experience, I've wondered if there was meconium on the bottom of the tub, but it was sediment instead.
Bacterial concerns are the ones where the water is not safe to drink, brush teeth, or bathe in - the water on your toothbrush or going up your nose can be enough to get you sick, especially if you're immune compromised. You shouldn't labor or birth in water that has this type of bacterial contamination. Bleach or iodine could be used to kill the bacteria, but it seems like a bad idea if there were any other options.
Chemical contaminations of water - it can be okay (or not okay, depending) to shower or brush teeth in this water - ie, take in small quantities but not large. It all depends on the issue. Usually, certain chemicals are known to be a problem in a certain area - in my county, it's a raspberry pesticide that's harmful in very small quantities. You keep your mouth closed in the shower if you're in a contaminated area. Birthing in this water would be fine or not a good idea, depending on the contaminant.
Here in the US, you usually take a sample to a local water testing place, often the health department or university extension. They know the local issues to test for...I'm really not sure what the equivilent would be in Thailand.
I have never seen any evidence that salt would be helpful for reducing bacteria in a birth pool - at least at concentrations that wouldn't make you float on top of the water. The most common bacteria to contaminate water would be human bacteria - fecal coliforms or even strep off the mother's body (for people that use the tub over days or weeks without refilling). Those bacteria thrive in a slightly salty environment.
My calculations - human body -.9% salt
Birth Tub - 250 gallons = about 2000 pounds
One half cup of salt = 4 ounces
.25/2000=0.01% salt in the pool
Not going to hurt the bacteria at all. In fact, they might like it.
Forgot to answer the orginal question: in the absence of other information - I think it will be fine to use it, with the usual caveats - I don't think you should fill a pool with warm water and let it sit around for days at a time in the air. A freshly filled warm water pool is safe for day or two without anything to prevent bacterial growth, as long as it's safe to drink.
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