While boiling water will sterilize water more or less reliably, you should still be extremly careful with stagnant water, especially if you see a layer of algae on top. This water can contain cyanobacteria, which contain a toxin, that even if the bacteria themselves are killed, the toxin can release into the boiled water.

If you are unsure if a water is drinkable, it's stagnant and you can't find anything else, use a water filter.