I was thinking more along the lines of whether there is something like a continuous glucose monitor but for dehydration, which could beep or otherwise alert a semi-independent elderly person that they are becoming dehydrated so they could get some water before it started to stress their kidneys. Like those pill boxes with alarm clocks built in so they don't forget to take medication.
But I'm aware there are various issues with CGM's (a friend has one, which may act as an insulin pump as well, and complains that the needle needs to be moved around every so often as the attachment site stops giving accurate readings). The cost/benefit probably isn't there, if an elderly person cannot control their own hydration they probably also aren't able to adequately care for a CGM or similar monitor. If it was something you could implant into the body and poll wirelessly it might have some value.
I think you could get 99% of the way there with a reminder at 10am, 2pm, and 6pm to have a glass of water. With a few exceptions (e.g., heart failure), there's no real harm in moderately overdoing it--you'll just pee it out.
But I'm aware there are various issues with CGM's (a friend has one, which may act as an insulin pump as well, and complains that the needle needs to be moved around every so often as the attachment site stops giving accurate readings). The cost/benefit probably isn't there, if an elderly person cannot control their own hydration they probably also aren't able to adequately care for a CGM or similar monitor. If it was something you could implant into the body and poll wirelessly it might have some value.