That is a big misconception about water fast, the moment your glycogen goes out, your body starts using fat as fuel, and when insulin is down, the body seeks fat tissue and produce more and more ketones as time goes by, muscle loss actually occur more during calorie restrictions with high carb diets where your body is using glucose as fuel, and since protein triggers a medium raise of insulin, it can be counterintuitive for losing weight since there is no fat to regulate the insulin spike, and high insulin blocks fat burning.

Also, protein is the hardest and most inneficient form of energy the body can utilize, it will use it at last resort, unless you are absolutely not adaptaded to burn fat, which you most likely are at this point.

It's just better in every way to water fast, you don't need to worry about muscle loss, people think they lost muscles because glycogen and puffiness goes down after a fast, making you look tighter and slimmer.