I feel like drinking unpurified water out of desperation is a tale you'll eventually hear from people that've been outdoors a lot. It will either end unpleasantly or with extreme emphasis on how lucky they were that it didn't end unpleasantly.
Dying of dehydration is a whole lot faster than giardiasis. If you're in a survival situation and are reasonably sure of a rescue in less than 14 days, go ahead and drink that water! Otherwise, don't.