Any Nihilists here?

[The Grand Nudger]

It's objectively helpful whether they want the water or not.  The hunger strike example is a subjective objection to an objective fact.  It's not that the water is harmful, they don't want it.  Their rejection is a statement about them - not water.

As long as there are statements about them which change whether it's good or bad, then it isn't objective.  Who is it helping if it isn't helping them?

Any objective moral system has to respect change in factual details.  Otherwise it wouldn't be objective.  Subjective and non cognitivist moral systems are the ones that don't have to respect those changes - because those details aren't the basis of a subjectivist or noncognitivist assessment.  

Giving a person dying of thirst water is objectively helpful.  It's why there's a duty to care even when prisoners go on hunger strikes.  It's a good example of exclusively subotimal decision fields too, though, because in the process of saving their life we might override their own wishes and..arguably, reduce the effectiveness of their protest.  So, in this situation, you're stuck with two bad options.  In one of them you help and the guy might get angry with you, in the other you watch someone die when you could have helped. I'd say, at best, if you select the second option, you're both morally and practically apathetic. At worst, you might be personally culpable for their death.

-mind you, that's in a limited hypothetical. There's usually the option of giving a person on hunger strike what they're asking for. Sidestep the whole mess. That'd be where my mind went first.