So after testing over the course of a tank of water (we pump up and irrigate from this tank so has moderate turnover) i found it was not reading more depth than about 80cm, after much debugging ive come to the understanding that the ultrasonic control board and sensor are very susceptible to interference, thus why the cable for the sensor comes as a shielded coax. So it was not a good idea of mine to put un-shielded wires on the ends, and also to bring the wires out the 'front' of the board rather than out the back as it originally comes.

Also i found that with NiMh cells once it got down to about 2.1v overall that when the sensor control board is powered on , it causes a momentary voltage drop that is enough to vcc down below 1.8v for a moment which trigger the brown out setting and the arduino reboots, so 2x NiMh will not be able to be used right up, but this is not too bad, otherwise use alkaline.