Dehydration will very quickly reduce your ability to undertake the activities required to survive and so finding and conserving water should be a very high priority in any survival situation.
Clearly there are potential dangers associated with drinking contaminated water but these need to be weighed against the dangers of dehydration. As with most survival decisions this is based on a balance of risks to the best of your knowledge.
If at all possible you should find the safest possible source of water and your basic outdoor kit should include several methods for treating drinking water but if it comes down to a choice between drinking iffy water and dying of thirst then the answer is obvious.
The only exception to this is when you suspect that the only water available would do you more harm than good. Salt water or anything which is obviously grossly contaminated may just speed up dehydration, many water borne diseases cause diarrhea and vomiting which will just dehydrate you faster.
Ultimately this comes down to your preparation, both in terms of the equipment you have with you and your knowledge of the environment you are in and planning for finding water in an emergency should be part of your preparation for any expedition.
Similarly if you are unsure about the water you have available it is sensible to hold off drinking it until you do actually need to.