I would like to know whether there are sensors to measure the depth of an unknown area, Its better if it locate above the water level. If not I can submerge it.
Water pressure is proportional to depth, so a pressure sensor can be used.
You can buy ultrasonic depth gauges, but the transducers have to be underwater.
Yes - they are called depth sounders. They use ultrasonic (usually) transducers to get an echo back off the bottom and calculate the depth. Depending on the frequency used, some have better range (lower frequency ones) while some have much better resolution in showing things in the water (fish etc.) - those are the higher frequency ones - usually somewhere in the 200khz range. You have to be very careful measuring depth like that (I used to have a digital readout one that was always getting confused). You will get an echo back from anything that is a different density - the bottom will give a good echo. Unfortunately, if you are in salt water near a river for example, the density change between the fresh water floating on top and the salt water below will also give you an echo and a false "shallow" reading. Same effect with a thermocline.