There are a few DIY guides out there.
I don’t think a bathtub is the way to go. They aren’t long enough for most adults to float in. Plus they take a lot of salt, so either the bathtub is dedicated as a sensory tank or you have to drain it / refill it frequently (which gets expensive) in between using it as a normal bath.
The salty water can go down the drain though.
My keys, which is the reason I got a cheap detector. I haven’t used it since, but have loaned it out a couple times for other people looking for their keys.
A small child nicked my keys and dropped them somewhere in a large field. A detector was cheaper than a new electronic key for my car.