Coconut water should not and is not supposed to taste sour and if the coconut water tastes sour then it has gone bad and should be dumped out.
Coconut water is supposed to have a coconut-y and sweet taste to it and when the coconut water has gone bad then it begins smelling sour which means it is not safe to drink.
Although it's not too bad for you to drink soured coconut water it still could make you sick so I would not recommend drinking it.
When you're not drinking coconut water it should be kept refrigerated and not be left out at room temperature.
If in doubt about the coconut water or anything else being good then just throw it out to avoid risking your health.