Simple, can be done with a few transistors and a piezo beeper. Finding it, you need to wear really big earlobes because sound is faster in water, and sounds like its coming from in yourhead. But the big ear lobes correct that so you have asense of direction of the sound. I can't think of a simplerway. You said pool but I am imagining more water than that,for a lost boat motor. In a swimming pool this is useless.

What about rain?

For a boat, you can simply tie a float to a string. But for an individual or other gear the sonic whistle is pretty good. The battery won't last too long because the drain on the battery is pretty high. So a person would have a very limited time to go searching for whatever was lost.
Another concept would be to have a high tensile strength rope hooked to the top of a boat motor for instance and have a water activated CO2 cylinder inflate a float. The rope would be laced in such a way and coated with starch for instance so that as the starch got wet it would release from the motor and the float would take it to the surface. That way the boat motor could be hoisted directly without having to go diving for it.

Also, whatever device is used could be put in an ultraviolet resistant plastic bag. When a small depth sensing device (cheap) senses the device is deeper than 3 feet under water, the device is deployed.