If you talk to anyone involved with fasting and mention a water fast they will understand that to mean fasting while still consuming water. If everyone else agrees on how the term is used then you may want to reconsider if the term is wrong or not.
That might be a stupid question but is there a grammar rule or something that would suggest that "water fast" is "abstaining from water" and not "abstaining from food except water"? Maybe it's an expansion of the definition of fasting and thus leads to "abstaining from water", in which case the correct term would be "water diet".
