Lasers for finding water depth

[inventor73]

I am hoping to find a laser or electrical instrument that can be held 1-4 feet above water and can be set to determine or signal when the water depth is more than 4 inches above the surface. The instrument must be relatively small in size, preferably the size of a VHS tape or smaller.

PLEASE HELP IF YOU KNOW OF ANY DEVICE....THANKS!!!

Any suggestions or ideas are greatly appreciated.

Also, if you know of such a device, what would the cost be to purchase.