Sounds more like Egg yolk peritonitis to me. Water belly would be more water balloon. EYP is firm and very little give when you push. I had a hen with EYP she lived months before she's got to the point she couldn't move her legs because of the mass. I believed it to be ascites but after attempting to drain nothing came out. I also opened her up after I put her down. And indeed was EYP.
She already can't walk much at all because of her belly, but bless her heart she tries.
In the early stages of EYP you can treat with antibiotics and I've heard of it being successful. But it isn't a cure. The problem is the hen is laying internally. I don't feel there is much to be done for her at this point. The kindest thing you can do you is to end her suffering.
To be honest I don't know. She certainly eats a lot. I'm fairly new having chickens, 4 yrs, and I am totally clueless about something like this. For those years I haven't had any serious conditions like this.
