Hi - I feel your frustion - I am dealing with a mil very similiar for 2 1/2 years and honestly I don't think mil will ever change or accept that the "new" methods of raising children are proven to be better and children need boundries - you know the saying "you can't teach an old dog new tricks". My husband and I finally decided to stop the visits at Grandma's due to safety issues. It was really a tough decision and we had to find other free ways for childcare needless to say I don't get enough breaks as a mother should.
These are some of the pains we dealt with - I could go on and on and vent - yes I'm still angry and frustrated.
Our daughter was not supposed to have peanuts or peanut butter before the age of two but my mil gave it too her and acted dumb likes it no big deal and we would tell her all the time do not do this.
I have asked her not to give M&M's before meals and right infront of me and others the mil gave our daughter a big cup of m's - the mil later wondered why my daughter would not eat the hotdog for dinner and I announced infront of everyone at the party that maybe it was becuase you gave her M&M's when I asked you not to. That one shut her up for a while. :)
I too had the water issue - I had just gotten our very stubborn daughter to drink water and it took almost a month of weaning onto water with a smaller amount of juice each time. I was livid and of course my husband did not understand the frustration becuase he was not home trying to deal with this. My mil still gives her juice all the time even when I told her not too on DR orders because its empty calories and he wanted our daughter to be hungry and eat a nutritional meal so she will gain weight - even with an explanation and Dr orders she does not respect what we ask.
My husband has talked to her several times and it goes in one ear out the other.
I have found I have a better peice of mind if we keep our visits very short and away from the home. Good luck!!!