Hi, I'm new in this forum, thanks to everyone for this help forum.
I want to measure distance between me and any object, and I want to do this in water. I'm not good at English so I have a drawing.
How can I do this? Which sensor can I use? Distance range I want is 15-20 cm. Please, help me 
Sonar is typically used for that. Underwater sonar transducers are not cheap, be warned. For that
very short range perhaps there is a chance the water will be clear?
If so IR distance sensor would be a cheap option - not exact, but you haven't said how accurately
you want to measure distance.
With a sonar device, there is a chance that the transmitted pulse is too long and will overlap the echo. Check the spec data carefully before purchasing the device.
You also need to make sure the time between the bursts are long enough that you are not seeing something that is a multiple of the distance you are looking at as "too close" (if the echo from one pulse comes back just after you send a second pulse, your system will think it is "too close" ).
Dear Friends,
Thanks for interest and replies. I found a solution, there is a sensor which can measure under the water. That's url: JSN-SR04T Su Geçirmez Ultrasonic Mesafe Ölçer Sensörü (Transducer) Uygun Fiyatıyla Satın Al - Direnc.net®
Thanks..
I think that sensor is meant to operate in air, not water. It's waterproof, not submersible. So even if it survives at your operating depth, it may not work for you, given the different speed of sound in air vs water (and impedance).
