I have a utility pump I leave on the floor of an area that sometimes gets a few inches of water. Right now I have to turn it on manually. I'm looking far a sensor that will turn the power ON when water is detected. There are plenty that will turn power off but that's not what I need. Thanks.
They make pre-made consumer products specifically for this, that are approved as a complete package, and work reliably, and don't require any kitbuilding or hardware hacking. They are called sump pumps.
The key to success of a sump pump is to have what you should have anyway: a sump. A small hole at the lowest point where water tends to collect. The sump can also have drain pipes added to collect water from other areas of the floor and piped into the sump.
The hole will have some depth of water in it before the floor has any depth of water. That gives the sump pump an advantage, as it can wait until there is some depth of water before starting up, i.e. making it worth running for.
The sump only needs to be large enough to fit the sump pump inlet and float. However, the larger and deeper it is, the less often the sump pump will run.
I bought a utility from either Home Depot or Lowes, I do not remember which store but they both sell similar products, that will turn on at about 1-3/8" and turn off at 1/4" of water. The pump is a Utilitech model 0435062. It also has hook-ups to different size hoses. Check it out. This may be what you are looking for.