Thanks that is a good tip - I will try changing all the water containers in the morning, though from observation they seem to prefer icky muddy water but I will definitely give it a go.We're in winter at present and it's a relief to not worry about hydration.
I found Mary drinks more when the water is fresh, so in summer I empty and refill the water dispensers every day. If you're not doing that, you could give it a try and see what happens. Also, if they're earing ice, maybe they won't feel thirsty?
And yes, I am hoping they are eating enough ice, but it is hard to tell because of course it melts while they chip away at it and then they get to the cucumbers and chow down on those (lots of water in those too of course).
I won't fret too much as they seem very active and busy. The new game is 'chase the little one' which looks way too energetic to me in these temperatures!