The MB70xx series isn’t rated for immersion but rather water resistant (IP67 standard). I had an application for something like that - a cistern used for irrigation fed by a spring. It was useful to know the level of the tank. The narrow beam avoids false readings from the scatter echo of the sides.
I’m still puzzled by the seemingly incorrect ranging numbers returned by the device.
As Philba said, with “for water tanks” I mean that they are often used to inspect the water level in a closed tank.
Their narrow beam is designed tipically for this kind of application.
It is waterproof, so it can work with water vapour (it is not unusual to see vapour in tanks for high temperatures).
Maxbotix answered me by explaining that these sonars has an internal memory which store the last detect distance if nothing new is present during ranging; however, me too, i’m not able to explain why my devices output random values. I think it’s because their narrow beam.