Coconut water is good, just have to be fresh.
Yes.... the taste turns bad after about 60 minutes exposed to the air. Should be taken fresh. Coconut water, which comes in packets also tastes bad. They add all sort of preservatives, giving it a weird taste in the end.
Will it taste ok straight out of the cocunut from the supermarket, or do you literally need it fresh off the tree? Coconut milk is meant to be a lot nicer.