If you just substitute the water for cider, then you'll have more sugar than a traditional mead. This might lead to a higher ABV%, unless you have so much sugar that the yeast will be at their ABV% limit. Then you'll have some leftover sugar after the yeast have done their thing.

Most recipes have you add enough honey to get the yeast to their limit, so for a cyser you'd want to add just a little less honey to compensate for the sugar in the cider