It's likely a way for a judge to inject their own subjective judgement into an accusation of witchcraft: The hope is that people that are innocent are more like

It's likely a way for a judge to inject their own subjective judgement into an accusation of witchcraft: The hope is that people that are innocent are more likely to accept a "trial by water" because they hope the deities will help them survive that trial, and the judge can then also add a little of their own subjective opinion, after talking to both parties, to either say "yup they survived the trial, pull them out of the water" or (if they think they deserve it, for whatever reason) let the person drown.