Thankyou, everyone. Cats are clearly much more complicated than dogs.
@sashh - I'm sure you're right. I haven't put water down for my adult cat for years. He viewed it with disdain, so I stopped bothering. I do put down his weight-control kibble twice a day, which he grudgingly eats. I'm reasonably confident he's getting fed at half a dozen other houses, because he's not getting as big as a bus on weight-control kibble alone. Maybe one of them has acceptable water?
The kitten room is currently the spare bedroom. There's a babygate on the door, so they can rampage roam through the whole house, but the dogs can't raid their litter tray bother them. It has four corners: food bowls, water bowl, litter tray, and climbing tree, respectively. There are no more corners. If they want a decorative water feature, they will have to wait til they're old enough to go out. And find someone with a garden which has one, because I don't.
Watchet (the one with the thumbs) is clearly desperate to go out. I'm not sure she knows what 'out' is; it's just the lure of the forbidden. I'm having to be very careful with her and doors. She did, in fact, slip past me and get out once, but let me pick her up and bring her back in again. Whitby is basically, "I'm sure it's nice out there, but inside has dreamies..."
Whitby has decided 2am is cuddle time. She told me this by rolling on my face and purring loud enough to wake the sun. Will this get better, or is it one of those things I just have to get used to? Adult cat doesn't come upstairs - his choice, there's nothing to stop him, but he basically just moves from food bowl to sofa and back again - so I'd forgotten how noisy a 2am cat is.