After a little discussion in the comments, I'll put forward one suggestion.
I'd recommend looking for something like this (on amazon) for in front of the sink. It's a memory foam, non-slip bath mat and they're sometimes on offer at places like Costco (UK), so I believe cheaper models are available. That one is fuzzy, but I've seen an equivalent that is more rubbery and smoother (more like wipe clean). In my experience, they're like big, heavy, rubbery sponges. They're quite thick and actually quite fun/comfortable to stand on, so it might provide a nice distraction, too.
It'll (hopefully) do two jobs:
- It might prevent the chair from being moved in front of the sink. If she's dragging the chair rather than lifting it, the non-slip and slightly higher profile of the bath mat will make it harder to get it in front of the sink (buying you time to catch her moving the chair).
- If the chair makes it into position and the water flows, it'll catch at least some of the puddle. Because the mat is spongey, do be warned that it might make the chair less stable.
I'd also recommend "rug gripper tape" to keep the mat in place if it's too easy to move (in my experience, they have rubber backing and are quite "solid" so you might not need this), and I'd install the mat when she's not looking and say it's for mummy/daddy's comfort on their feet when they have to stand at the sink. I'd probably waddle over it a few times for fun at this point, too - I genuinely do like the feel of these things!
And, I'm sure you've already done this, but check the thermostat on your hot water is high enough for comfort but low enough for practicality.